the tag lowsrc="bubblelo.gif" (stick it into your IMG tag) will load bubblelo.gif first then bubble over the top, replacing it.
Generally, this is provided to allow for a preview on slower connections, but can be used in many tricksy ways. If you had, say, a page full of thumbnail pics, you could use a single low-src image that has your logo and the word "loading", perhaps with a little animation. It does look pretty cool considering it is a very basic HTML trick.