Where possible I use SVG for diagrams and the PNG image format for photographs (including APNG for animations, now at last converted from animated GIF format). 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.
I use images published by other websites by reference, in moderation; where I do so, I link to where I found the original. If you you own the copyright to such an image, or are responsible for the web-site that I use this way, and wish I wouldn't do so, please let me know; to the best of my knowledge my use is covered by fair use provisions, but I have no problem with ending that use.
The remaining images in use on this web-site are published as part of the site. Some of these I know to be public domain, some I have derived from such and some 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.