@ Iowah Munters
Hmmm… that would make it seem that their is an issue with the install and or configuration of the GD or Imagick libraries on the web host. Or, the program you are using to save the JPEGs are creating invalid files. Another possibility is the files that were being uploaded were too large (pixel size, not file size) and when the server opened them to resize it was running out of memory. Other avenues for you to explore if you wish.
Hope that helps!