If you just want to resize only one file, then gimp would do fine. But if you want to change the size of lots of pictures then you'd better use something from the command line. If you use TeX you probably have a program called convert. It is not released under the GPL, but you still can download it free. With convert you could just
convert -resize 100x100 image1.jpg image1-tiny.jpg
and get a REALLY tiny version of a file selected. There are of course more options ("convert -help").