Hi Ian,
As Mary has already pointed out, Moodle did not start using its own caching until Moodle2, so as you say you are using 1.9, it is not going to be related to any Moodle cache other than what is in the browser you are using.
However, I'm not sure that the issue might not be something a little different, given what you report below with the format type error.
Over the last few weeks I have had a number of jpeg images not being recognised by IE for some reason (which I have not got to the bottom of yet!) but resaving those images in my graphics package as .png files resulted in the image loading correctly - this was on both Moodle and on a standard non-cms based webpage, but using IE9. I don't know if the issue has been with my IE, or with my graphics package not saving the images correctly in some way, but it might be worth giving that a try??
HTH
Richard