an art instructor who charges for his courses is using one of my photographs in his class. all the students including himself have painted the photograph and he has posted his painting on his site. I did not give permission to any of these people to paint my photograph. this is my most popular photograph which I sell in many shops in the area (I live in Canada). Is there any infringements of my copyright in this case? thank you for any information on this
A good place to start to gather some information about this issue is the Wikipedia entry on derivative work, i.e. creative work that includes important elements of some other copyrighted work. Most of it covers US law and concepts common to several countries, but there's a brief section at the end that covers Canada specifically.
For some clarity (and perhaps a way to discuss the issue with the art instructor in question), consider how a painter would respond if a photography instructor had his or her students photograph a painting. A photo that includes all of a painting and little else might be more likely to offend the painter and infringe his/her copyright than a portrait of someone that happens to capture the painting hanging on the wall in the background.
By UK law that would be an infringement of copyright if the work is clearly recognisable. If no permission was given (or maybe it could have been implied) you could ask for a fee, for the painting images to be removed, or you could sue which would most probably involve you paying some court fees upfront (and these could be claimed back if you win). Research your local laws.