Im guessing this is a Linux/UNIX server.
If so, you will need to CHMOD the image files. I forget all the apropriate permissions, but something like CHMOD 755 <filename> will do the trick. If there is a system admin or someone like that, they may be able to help you out with the exact CHMOD that will suffice on a web server. If not, there's always wiki.
http://en.wikipedia.org/wiki/Chmod