Where possible I use SVG for diagrams and the PNG image format for photographs; I intend to convert the few animated gifs to the APNG format, eventually (but the urgency of that passed when the patents expired). Most of my SVGs are geometric illustrations, typically used by my pages on mathematics, e.g. the statement and proof of Pythagoras's theorem (see margin – best used as a background image, so that it tessellates). These are my own work – please feel free to recirculate or reference from elsewhere, if they're what you need.
Various pages include images I know to be public domain, images I have derived from such or images I have created myself. I use my geometric illustrations (in SVG format) in too many places to list here. The other pages using images not listed here are:
The W3C validator and CSS validator incite me to announce validity of my pages using their icons. Although their suggested HTML does use the icons taken from their web-site, I've made an educated guess that they're fine with me copying the following images here and saving their server some traffic:
I don't own the copyright on the images listed below and don't know who does: if it's you, please tell me and I'll gladly give due credit, or stop using them if you prefer. That applies to images received attached to widely-circulated e-mails as well as the following:
Finally, I use these in page livery, albeit (aside from the first) less so these days. If I remember correctly, they originate from the CERN web server, back in 1994, with which they were packaged; I take it they were so with (at least tacit) permission to reuse.