You probably uses an image because you like it, if you like it you should thank the photographer somehow. And the best way of thanking someone is to put credits onto their images, even after you have created a new piece of art with it.
Reason #1:
Photographers are human beings just like you and me, many photographers are taking pictures as a hobby, other see it as a future profession and others already has it as a profession. Photographers just like any other business need publicity, to get more costumers - in other words more things to photograph. And the hobby photographer may need more people to see their images so they can get more constructive criticism and become better photographers. If you add credits on the images other people who see your manip know where to find the image and maybe use someone else of the photographer's images. Or the viewer may live near by the photographer and have something they want to have just as good pictures of. You'll never know, but if you put credit onto the image it may lead to the photographer uploading more stock images and how can you say no to that?
Reason #2:
Imagine 5 years from now, when you clean your computer and come across an image that is really pretty, but it's text all over the place, because you might have been "bad" at making art back then. With credits on the image you can find the same images again and reuse them and put them together once again but make it better this time. Without credits... Well you shouldn't use it at all and if you want to remake it you have to find the very same images again and as we all know, Internet is rather big.
Reason #3:
One of the most important reason is to prove that you aren't an art thief. If a manip or even a picture has no credits onto it, per defination it means that you, as the uploader, is the one who has photographed it. If you haven't and the photographer doesn't know you have uploaded it, then you are an art thief. With credits on everyone can see that you aren't hiding anything, the image is legal. If you for some reason find a photographer you know has died and decide to use their images it's still art theft, because the images belong to the persons relatives and will do so until 70 years has passed between the day they person died until the images are free to use for everybody. But of course if you want to use a vintage photograph that is more than 70 years old you're free to do so.
Reason #4:
Might come when I have thought a little more about this x)
How do you credit then?
First of all, if the rule is that credits must be on the image itself you must make it visible on the image - you must make it able to read it, duh! Use a font that is easy to read in a size where you can read it. Preferably write out the whole url to the induvidual image because a photographer may have uploaded 600 images and that's just too many to search trough. However if you uses many picture from the same person just the (user)name of the person and the name of the site is perfectly fine. You can also add other info on the same place. Maybe you want to have some credit's as well for making a manip to someone else.
Self credit
As Reason #2 said, image a few months and years from now. You might find a manip you bought from someone in the past. You might be interested in a new one but can't remember who made them. Wouldn't it have been just awesome if their ID or something were written onto the image so you could find him or her again?
Other things to credit
Most people don't think about this, like, ever. And the programs we uses are working against us on this point too. Fonts and brushes are two thinks many people uses but the creator of those elements are seldom credited. I have to admit I'm pretty bad att giving credits to font creators. But it's always a good thing to write it up onto the image, at least the name of the font. Because you might remove it, forget you have it, change the computer and at some point a few years from now someone ask you what font that is and you are just clueless if you haven't wrote it down somewhere. Same thing for brushes.
Reason #1:
Photographers are human beings just like you and me, many photographers are taking pictures as a hobby, other see it as a future profession and others already has it as a profession. Photographers just like any other business need publicity, to get more costumers - in other words more things to photograph. And the hobby photographer may need more people to see their images so they can get more constructive criticism and become better photographers. If you add credits on the images other people who see your manip know where to find the image and maybe use someone else of the photographer's images. Or the viewer may live near by the photographer and have something they want to have just as good pictures of. You'll never know, but if you put credit onto the image it may lead to the photographer uploading more stock images and how can you say no to that?
Reason #2:
Imagine 5 years from now, when you clean your computer and come across an image that is really pretty, but it's text all over the place, because you might have been "bad" at making art back then. With credits on the image you can find the same images again and reuse them and put them together once again but make it better this time. Without credits... Well you shouldn't use it at all and if you want to remake it you have to find the very same images again and as we all know, Internet is rather big.
Reason #3:
One of the most important reason is to prove that you aren't an art thief. If a manip or even a picture has no credits onto it, per defination it means that you, as the uploader, is the one who has photographed it. If you haven't and the photographer doesn't know you have uploaded it, then you are an art thief. With credits on everyone can see that you aren't hiding anything, the image is legal. If you for some reason find a photographer you know has died and decide to use their images it's still art theft, because the images belong to the persons relatives and will do so until 70 years has passed between the day they person died until the images are free to use for everybody. But of course if you want to use a vintage photograph that is more than 70 years old you're free to do so.
Reason #4:
Might come when I have thought a little more about this x)
How do you credit then?
First of all, if the rule is that credits must be on the image itself you must make it visible on the image - you must make it able to read it, duh! Use a font that is easy to read in a size where you can read it. Preferably write out the whole url to the induvidual image because a photographer may have uploaded 600 images and that's just too many to search trough. However if you uses many picture from the same person just the (user)name of the person and the name of the site is perfectly fine. You can also add other info on the same place. Maybe you want to have some credit's as well for making a manip to someone else.
Self credit
As Reason #2 said, image a few months and years from now. You might find a manip you bought from someone in the past. You might be interested in a new one but can't remember who made them. Wouldn't it have been just awesome if their ID or something were written onto the image so you could find him or her again?
Other things to credit
Most people don't think about this, like, ever. And the programs we uses are working against us on this point too. Fonts and brushes are two thinks many people uses but the creator of those elements are seldom credited. I have to admit I'm pretty bad att giving credits to font creators. But it's always a good thing to write it up onto the image, at least the name of the font. Because you might remove it, forget you have it, change the computer and at some point a few years from now someone ask you what font that is and you are just clueless if you haven't wrote it down somewhere. Same thing for brushes.