|Make sure all images have the correct file extension. Accepted file extensions for images are .gif, .jpg and .png
(these are the image formats which most browsers can display without needing extra plugins)
Mac OS up till OS9 does not need file extentions to associate files with the programs used to view them so Mac users should take extra care to add the correct file extension when transferring such files to any other operating system.
The scripts on the server need to have the file extension to know that they are images.
Also - make sure you use sensible file names that all operating systems can handle. Do not EVER use spaces in file names and stick to alphanumeric characters (apart from the dot before the three character file extension).
The web server we use is FreeBSD (unix) so please also note that file/directory names are case-sensitive. (windows/dos users should take special note of this!)
I haven't yet seen Mac OSX but my guess is that its naming conventions are probably fairly unix-like so provided you don't use spaces and use the correct extensions you should be ok.