using these images
Images up to the standard resolution shown on this site (i.e. a maximum dimension of 520 pixels on the longest side) can be used free of charge for non-commercial purposes under the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. If you're not familar with Creative Commons, the link above includes a brief plain-english description. Attibution must include a link back to this site so that others can find the original image.
For example you could use something like this:
As long as you follow the licence, you're welcome to use the images for free as much as you like, on non-commercial web sites, publications, blogs, or whatever else you choose. Please respect the effort and care I put into the quality of my images and don't upsize them from the resolution displayed here, or print at less than 300 pixels per inch, either of which would significantly reduce image quality.
commercial use and larger sizes
Permission to use these images commercially or at larger resolutions is available at reasonable rates. Please contact me for details.