this happened to us (UniWiki, Tiki 1.8.4, with .jpg's), and we concludded that it was an issue specific of the .jpg file. This seems as if it depends on the version of jpg (although all jpg should be the same, isn't it?). It happened the same to me years ago with .tif images, generated from different programs, and not all of the generated the same tiff image, even if they said they did.
Suggestion, convert the image to other formats (gif, png, ...), and upload again. To convert, on MS Windows platfrom, I recommend the small, easy, powerfull and free program called Irfanview http://www.irfanview.com. At the "save as" menu, you can chose image format, ...
(report if this helped!)