For some reason, every time I try to upload a picture onto a picture hosting site like photobuket or imageshack, it always comes back with an error "file not uploaded." I've tried saving it with different programs, saving in different formats, and uploading onto different sites, but nothing works. It worked like 2 weeks ago, and now it just suddenly stopped.

Reinstall browser/use another browser.
Update flash player.
Check if the files are virus infected.

What browser + add ons?

I'm using firefox 3.6 and the only add-ons I have are some java extensions and a firefox background.

A Firefox background?

And though I've never had this problem, I've experienced something similar. Occasionally, something will make my Firefox crash whenever I finish selecting a file to upload to Imageshack. Much as I hate to advocate such a behavior, I'd suggest using Internet Exploder for image uploads only until Firefox goes to 3.7; then check if it's safe and functional for uploads.

(Think my crash may be due to a Flash trick Imageshack uses for their file uploader, but whatev.)

Chrome works, use Chrome instead of Exploder.

For the record, I'm using Firefox 3.0 and have never experienced such problems while I heard of people complaining about problems with FF 3.5+. Sometimes upgrading to the newest version isn't the best idea, especially if the product you're currently using works perfectly. "Never touch a running system."

Firefox 3.6 was majorly gay for me on winxp. It would always corrupt form data, which is why I asked. I did a reformat with win7 and firefox seems fine now.
You could try uninstalling and reinstalling firefox or use chrome.

I'm using the current version of Firefox and have had no problems.

Also, you could try uploading a sig image on SEN and see if you can do that.