are people still having problems uploading? The Call to undefined function ImageCreateFromJPEG() in... was a system problem due to some of the server updates which has been resolved. (Not sure how long it was broke, a few days maybe).
Not sure what would cause"Sorry, we only allow uploading GIF / JPEG / PNG images" problem, except maybe a bad image. Can you view the image in a web browser?
PS: When using internet explorer, I copy the image to the clipboard (right-click and say copy), just use PAINT, and then paste it, next pick file, save-as, set the type to jpeg, and put in the file name. Then upload that.