Cant upload pictures to site using custom fields

Hi

As part of my payment process customers need to be able to upload 4 pictures to me. I have set the allow files but still get internal server errors.

I can upload doc, png files ok but not jpg files they seem to be a problem but need them as most people upload with jpgs.

My server has been set up to receive large files and with large time out times so i cant understand what can be the problem.

Iv contacted my server provider but just to confirm do you guys think this is to do with my server and not maybe the Hikashop itself?

Many Thanks