TeX Preferences
What flavour of TeX would you like to use?
Plain TeX
AMS-TeX
LaTeX
AMS-LaTeX
Image Preferences
Where should the image be stored?
Store the image on the server.
Put the image directly into the HTML. (Unless it's more than 8 KB.)

If the image is stored on this server, then it will vanish if this server is turned off or taken offline, or if my script has a bug in it and loses your formula, or if the Internet is being flaky. In addition, the image will take a little bit longer to load.

If the image is stored in the HTML, then there will be a couple kilobytes of garbage in your web page or Livejournal entry. For example, a relatively complicated image like this is 2.5 kilobytes. Internet Explorer cannot handle this type of image.

LaTeX Preferences
Which LaTeX package(s) should be used? (This won't affect anything unless you're in LaTeX mode.)

Some commonly used packages:

"amsmath, amssymb" gives you the AMS extensions (like \mathbb)

"mathrsfs" gives you curly calligraphic letters using the \mathscr command.

"beton, eulervm" sets up the Euler and Concrete Roman fonts.