After doing a WordPress 2.6 upgrade this weekend I found that an issue I have uploading images is focused to Macintosh. WordPress is a server side software application that runs this website.
There are several web post like these (assuming they are on macs)
WordPress › Support » 2.6 Image Uploader – Will Upload But Not Insert
I opened up Parallels running XP and Firefox and the problem DID NOT exist.
(I have the latest Flash 9)