I'm having a similar problem, except only some users are having trouble. And the users that are having trouble have been able to post pictures in the past. Now they are getting Invalid File when trying to upload an image. I've checked to make sure the image file types are correct and that the sizes are fine. In fact, I even took the image that one of the users tried to upload (and received that Invalid file message) and I was able to upload it without a problem.
I just went in and ran maintenance for the boards and the gallery. So we'll see if that helps.