I use png format where possible (the exceptions are anmiated gifs); eventually, I'll link to the FSF's page explaining why ... I also (since the middle of 2005) use SVG images, since I can write my own.
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.
Aside from the images I've had attached to widely-circulated e-mails,
me, dancing at Suffolk and Goode (1998, I think)
me, juggling at craphenge
a little version of that
so good I pilfered it
statement and proof of pythagoras' theorem
(best used as a background image, so that it tessellates).
This one is genuinely my own work - please feel free to recirculate.
I use these in page livery, albeit less so these days.
Written by Eddy.