IE 5.x for Mac OS X has some problems.
The problem you are facing is that ALL Macs running OS X are Apache Web servers. Mac users have a shared directory. They have to place their image in their shared directory and turn 'File Sharing' on. This turns on their Apache Web server and allows the browser into their Shared folder, or the the same as a Web root folder on their Web server.
Because of Mac users whining about OS X being too much like Unix, Apple covered it up with Mac terms for Unix activities.
Your answer is in explaining to your Mac users how to get their files in the right place and how to allow your 'Browse...' button to access that file.