Thank you for your response. The good news is that I've fixed the problem; the bad news, if you can call it that, is that it was fluke rather than one of the suggested methods!
I tried all the methods up to the fifth one, where I started to come unstuck ( when I clicked 'rename', it just let me rename the file with the .old suffix, rather than having to change it from .jpg; also, there was no slideshow.ini file shown ( I've checked, and there still isn't, although I'm now running a slideshow ). In fact, the solution was as simple as changing the setting in 'personalisation' from 'stretch' to 'fill' ( although I had to re-select all the images to use in the slideshow ). Quite why a power cut should cause this particular effect, I can't imagine ... but luckily it's sorted now.
Thank you again for the suggestion, though.