I had a problem recently where the loading window would just sit there and almost dare me to try loading anything! It would just go blank. I emptied my browser cache and it fixed the problem, but in the past I have had to change browsers. Sometimes there are days where the image uploader doesn't work and nothing will fix it, but it appears to fix itself within a day or two. I think they use a flash based uploader so it might be worth checking to see if your version of flash is up to date too.
