Speed testing attached images

I did a test with FireBug to test if leaving 'compress' checked when adding attachments to a document would make a differce in loading speed of a web page showing these attachments as images:

The test pages: compressed images and uncompressed images. And the result?

I notice no difference. How does the Domino server do that? Or did I use the wrong file types? BTW: these are very nice banners I found somewhere. Maybe one of them can serve as a header background for a blog?