The vast majority of my photos are released under CreativeCommons ByNcSa license:

creativecommons.org/licenses/by-nc-sa/2.0/deed.en

That means that you are free to use the image in any non-commercial venue provided that you attribute me, and that any modified forms of the image carry the same terms and attribution. If you are displaying the photo on web pages, simply linking back to the Flickr page is sufficient attribution. In static form (slides, print, etc) you can just attribute to "Charles Tilford".

I am not opposed to commercial usage as a matter of principle; contact me if you wish to use an image in a commercial product or for other forms of revenue generation. In most cases I have simply asked only to be attributed, but I reserve the right to make the decision in each case.

I'd like to think that these terms are fairly generous, particularly given that I assume most visitors would be considered non-commercial. If your need to use one of my photos is so urgent that you can not comply with the terms above (explicit agreement with myself for commercial usage, and/or failing to provide attribution), I also provide convenient boilerplate terms of $10,000 per year per photo, plus 25% of gross sales of any items sold that bear the image. For most people, however, it is likely most economical to simply attribute me as described above, and refrain from commercial usage unless you have explicit permission from me to do so.

On Flickr, links to detailed licensing information can be found under the "Additional Information" section on the right-hand side of the page.