Can't upload media

I’m on 3.3.1 using Prose, on my Mac using Lion, and Safari. I’ve tried using Firefox, and no improvement with the problem. In fact, I couldn’t even chose the version, above! Rebooted, up to date with all software.

So, the problem is intermittent. I go to upload- it doesn’t matter if it’s from computer, or already in the media library. I can drop files, or use the browser- same problem.
I put in a jpg, and the image appears on the first window. Check insert into post, and the next window is BLANK!! NOTHING appears. WHITE! Needless to say, the jpg does NOT get inserted into the post!

THis is driving me CRAZY!!! PLEASE help! I’ll be your best friend FOREVER!!
seriously – please send help fast!