Why didn't I think of putting this here before?
To put it in a nutshell, there are four conditions for use of any of my imagery on a web site (not including of course photos by other persons; in that case their permission must be requested).
1. That the use be strictly non-commercial.
2. That the work be properly credited.
3. That a clear link to www.john-howe.com
4. Last but not least, that the use of the image be in accordance with the image itself. This is perhaps hard to make clear, but I would refuse, for example, use of one of my pictures to promote ideas, concepts or undertakings with which I disagree, or for which the image's prime purpose is only to attract attention to something entirely different.
Obviously, I can neither control nor actually stop anyone from using what imagery of mine they decide to use on a site, but as I am generally dealing with people who appreciate the illustrations, I have run into few problems. (Most of the more amusing ones have been honest enough site owners who were "sold" my imagery by someone less scrupulous, rather like the Brooklyn Bridge...)