All of the following are the same photo saved from Photoshop in different formats.
Web optimised .jpg image (my usual method), then made to look shite by blogger compression
Standard .jpg saved at max quality with no optimisation, and then compressed by blogger
Saved as web-optmised .gif file, which blogger can't compress too much
Saved as web-optimised .png file, which blogger can't compress at all
Looks like the last three versions are all pretty similar, but I'm going to start saving my files in different formats and checking which works best going forward. Fundamentally I don't want my images compressed so likely to go with .gif or .png versions.