I think that the general CSS forum would be better for you, as Ralph says... that's where all kind of people "get together" and not only the students of CSS3. These forums are not seen by people that are not doing the course.
People will need to look at what's been done. We're not here to do your job but to help you understand why it isn't working and maybe we all learn something from it.
If all what you want is round the corners of all those images and you want them work in every browser... well, as you indirectly suggested probably the best method is changing the picture and make it rounded... and if you have photoshop, the software can do it automatically for you and that will take care of your rounded corners.
Personally, I don't care if it looks different in older browsers, as long as it looks nice and organized, so I'm happy if it looks squared in IE and rounded in others. That ensures me that I can use a CSS only solution.
Let a member of the staff when you have the code ready to move the thread.