If you find yourself on a machine without the semi-ubiquitous ImageMagick packages, you might at least have pnmscale. Starting with a PNG file, you can do the following to resize it to 450 pixels wide:

$ pngtopnm < ./firefox-upgrade.png | pnmscale -x 450 | cjpeg -smoo 100 -qual 100 > firefox-upgrade.jpg