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' 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.
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.
Aside from the images I've had attached to widely-circulated e-mails,
I use these in page livery, albeit less so these days.