When you create something, (like a song, a story, a photograph, or a painting), it is copyrighted by law. People cannot legally use your creation without your permission. But of course many people break this law.
Artists often think they can paint from published images if they just change the colors or move things around. And for the most part they can get away with this. But if you are truly an artist why would you want to?
For me the creating of art includes my own experiences. Taking my sketchbook and paints out in the field, studying from nature, and taking lots of pictures of the wildlife and animals I come across are at the very heart of my paintings. They become expressions of things I have seen for myself, and memories that I want to capture in my paintings.
This makes my paintings more valuable and each one has a story to go with it. I can share these stories with those who are interested, and I can take pride in my work, knowing I did not copy things from someone else. This makes my work truly unique and original.
Students learn from other's images, and that is acceptable, but when you start selling your work you are no longer considered an amateur artist, and this becomes completely unacceptable. Go the extra mile, take your own pictures, study the anatomy of your subjects, practice drawing to gain better skills, create your own ideas! Then find a way to let your clients know what went into the making of your art! This makes your work much more valuable.