You can use the convert program (included with imagemagick) to convert a LaTeX file to an image:

$ convert -density 144 -geometry 100% source.dvi dest.jpeg

You can easily create an .ico file (e.g. a favicon.ico file for your website) with the ImageMagick convert program:

$ convert myicon.png favicon.ico

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