You need to attack it from a different angle.
If you know how to use the language editor, you'll need to go hunting for the entry that puts the actual login text on the page. I'm not sure where it is off the top of my head...probably in Admin/Skins
Once you've found it, test by putting another word in and confirm that the text changes. Then replace the text with a cdata reference to the image file you want displayed in its place. It will look something like this below, but with the angle brackets replace by curved brackets - because the forum will otherwise make this invisible:
(![CDATA[(img src="/Portals/XX/skins/skinname/imagefile.png")]])
You should be able to hack it together from there.
Rob