I think making money from an exact copy is, of course, definitely copywrite infringement , but IMHO if you are using it for reference to make a different painting, then I think it's ok, especially if the subject you are using for reference, i.e. a tree or animal, is not exactly the same in your painting, then it probably won't be recognized anyway. The only person who can lodge a complaint would be the photographer or artist.
I have sometimes painted arctic wolves from Mexican wolves in a different setting, even the fur colours are different. It's the expression I want or the pose of the body or something as small as a tongue, teeth or feet. Sometimes it's a mix of various body parts from several wolves or the trunk from one tree painted into another setting. I sometimes have to change photographs to make better composition anyway, so they never are exactly like the pics.
Sometimes you can get permission from a photographer to use their pic for reference if you give them the credit for it when you post your painting. I've done that successfully before and sometimes they want money for it.
If you are just practicing and don't post it on the internet, who's going to care? Sometimes I copy exact photos for practice too, especially in newer areas I am working to improve. I post some of these on my blog under "For Display Only". (They are not all copies, some are just sold.)
