i believe its the same issue i as well as someone else has. when you make a post and go to "advanced," then "manage attachments," and that big window opens so you can choose pics to upload, you add some then click on "upload." the pics dont load, you get error message across the top of the window that says "not a valid image." even pics ive uploaded before with no problem do it.
That's exactly my problem. I cant upload any pictures from my computer or photobucket. I can't edit my sig either with a picture. I just opened a imgur account and it seems to be working that way. I notified an Admin and they confirmed the issue, apparently they did an update to the site (behind doors) and it is having the above issues.
So recap, upload from computer, no go (invalid file error) and so far for me "imgur" is the only one working.