There is no problem with copyright as far as we are concerned. I allow anyone who paints pictures from our site to sell their work as their own. The only thing I do ask people to do is should anyone enquire as to how the work came about to spread the word for us to enable others to do the same.
For anyone looking for their own reference material, Google have rolled out an update on Image Searching where you can filter your results by copyright use. When doing an image search, click “Search Tools” then choose the usage rights you need to an image.