hi broberts,
the actual version of ImageMagick seems to have a bug in converting, scaling or just in the JPG-handling.
the predecessor of the version 6.7.5-8, i guess it's 6.7.3-2, works fine.
tc@box:~/tmp$ convert -scale 600x480 '*.JPG' name%03d.jpg
convert: no decode delegate for this image format `P1010201.JPG' @ error/constitute.c/ReadImage/532.
convert: missing an image filename `name%03d.jpg' @ error/convert.c/ConvertImageCommand/3017.
no files are converted after this error messages.