qgis - Why are JPG files invisible in Georeferencer?

I want to georeference a jpg file (map) with QGIS 1.8.0 on a Windows 7 (64 bit) machine.

The georeference window opens, but the map is not visible. When converting the jpg file into a tif file (e.g. with gimp) everything works. I tried it with several files. Always the same: The tif files are visible and can easily be georeferenced, while the jpg-files are not visible in the georeference window.

Any ideas?

Thanks for answers, Ulli