Gilles,
it is one of those rare cases where there is trouble with the image creation. I don't think it is the temp directory's fault as the temp directory is used for the storing of the thumbnail images and the error you are getting is prior to that, in the 'get the real image and make a usable image for thumbnail creation' (imagecreatefromjpeg php function). This is a php error and there no way to tell what is the cause of the problem from my end. The php function returns 'false'. I will look into this function and see if I can make use of the documentation there is online about it and see if there is an extra step I need to do in my code to make this work for everybody.

Can you tell me what is your php version?
Just making sure you have php > 4.3

Also make sure PHP is set to 'allow url fopen'. I need to document this one, it slipped my mind.