File permissions gone on PC

There were 3 jpg files that were copied from a Mac computer to one of our shared folder on our NT Server running NT 4.0. From an IBM PC, we can view the 3 files on the shared folder but we cannot edit or open them in IE5.5 or any image editor. If we reset the privilegs of these 3 files before we copy them over to the NT Server, the problem is solved. What was wrong in the first place and can that be avoided?