Possible Rights Issue

Hello. Our students using their network accounts cannot do 2 things on their iMacs (Snow Leopard) that the teacher (using network account) can do: take a screen shot (it beeps but no file created) and preview images (blank gray page in document screen). I will run a repair permissions today but why would this happen on 12 iMacs? and why would those simple functions not be allowed?

What kind of networking environment? Definitely a permissions issue. Check with your IT Department.