Are you starting with a transparent b/g?(Sorry if that sounds patronising) otherwise you should be looking at saving the pic as a .gif (if you are using a lot of colours) or as a .png (if you are only using a few) .jpgs don't support transparent backgrounds. Alternatively you could try playing around with .tif files and alpha channels but that's likely to make the file size quite large.
(
Lord Gnome,
Sat 3 Feb 2007, 5:43,
archived)