An example: let’s say you buy a stock image of a kid eating a cone of ice cream on a background of a “school.” If you change the background in Photoshop from a “school” to (let’s say) a beach or a movie theater, that would be considered changing the image. Then you would have a “derivative” image in your hands. If such changes are done lawfully, as allowed in the copyright that accompanies the image, then it is not “doctoring.”

“Doctoring” an image is changing it “significantly” without the original creator’s or vendor’s consent. It is against the (U.S.) law since it violates the image’s copyright conditions to which you’ve agreed.

However, minor color or brightness adjustments, or cropping an image or placing a border around it may not constitute “doctoring” since they may not be enough to change the original image “significantly.” That’s a gray area open to interpretation.

It all depends on the copyright conditions of the image you buy or download and the kind of difference the modification makes in the original image.

Sometimes the copyright will allow you to change the image and make a new art work out of it and sometimes it won’t. It all depends on the conditions of the copyright contract you’re agreeing to.

If for example the image is in “public domain” then you can do anything you want with it except using it in an immoral or libelous manner.

(I’m not an attorney and you should always consult a licensed lawyer before arriving at any legal conclusions or making any business decisions.)