[rastermon]

If you have Photoshop, I would recommend using the .eps format. You can open it in PS, at any resolution because it is a vector based eps. The background is by default transparent. Saving as a png will give you a 8 bit alpha channel, (pixels can be partially transparent) as opposed to a gif with 1 bit transparency. The option of selecting a color to be transparent will not anti-alias to the color behind it (on your webpage) unless you had that color to begin with. 32bit png file will look good because the partially transparent pixels at the edge of the graphic blend into whatever color is behind it. make sense?
I would imagine GIMP can do the same, but I rarely use GIMP.