Artists maintain the right to protect images with current copyright laws.
I encourage my students to be creative and do their own work or try to "put themselves" into their work. Copious copying from a teacher or photo, or from a painting from a master adds to the learning experience but is not to be relied upon in a stand alone manner.
Borrowed compositions, borrowed ideas... no rather, stolen ideas, stolen compositions, colors are fine!
We may all work on a project together in class but you are in charge to make it your own. Classroom work is just that...
In my classroom everyone gets the same photo to work from and this photo is taken legally from a royalty free site.
You are allowed to copy the photo from these sites and render it artistically but not allowed to print it out in a stand alone manner and sell the copy. You can sell this painting from this photo and not pay tribute to the photographer.
Remember that thousands of people may have also downloaded this photo and are doing a painting from it. You are in charge to change it up otherwise you will have hundreds or maybe thousands of maybe similar renderings elsewhere in the world.
Otherwise, it is not legal or morally right, Without the photographers Permission... to do a painting from just any photo.
Art teachers also have a Right to protect their own work, as in their own photos and their own images of their paintings as they are also mostly, practising and career artists too. Its ok and normal and often required to set Boundaries.
And that applies whether you are a young artist, or a seasoned old one. Taking ownership of your work is ageless, whether it is a hobby or a career.