There are various [itex]\LaTeX[/itex] to .GIF generators online. For instance:

http://www.codecogs.com/latex/eqneditor.php
http://sites.hamline.edu/~arundquist/equationeditor/
If you don't know LaTeX, here's a bit of a primer (the above assume that you're ALREADY in math mode, you just need to enter your equation):

http://en.wikibooks.org/wiki/LaTeX/Mathematics