Have you tried various different jpegs, and images received from different sources? Occasionally (but admittedly infrequently) images saved as jpegs or Gifs have been through some kind of graphics software which re-encodes them and for some reason even if they display OK, can occasionally beoccasionally rejected by other software as being 'wrong format'.
That is highly unlikely if it is happening with all images from different sources.
If you would like to post one of these images (must be in a zip file) I can see if it will import OK into my CMS.
