Here's a quick Firefox extension I knocked up. It looks for images sized 256x192 in web pages, and replaces with a Spectrum-style loading effect. As far as I can see, it only works with the Firefox 3.6 betas, and not with FF 3.5.
Install version 0.2.0
New in version 0.2: loads the pixel data in greyscale before adding the colour data at the end. This is not genuinely authentic for a real Spectrum loading effect, but it looks a bit closer.
Demo page for other browsers which support "canvas" (tested on FF3.5)