Thanks, both of you. The first image I tried to load was well under ten megabytes and looks perfectly OK on my blog. It rejected it because it wasn't a "valid path"... which is weird because it's a jpeg, and loads OK elsewhere.
My full sized one was too big and refused to load, as I thought it would. but I'm not "fixing" it to be smaller because of loss of qualityof the original... I'd have to rename a copy and it has already rejected a renamed and reduced size copy. I just wondered if it was the underscore and alphabetical letters which caused the problem.
It looks fine on my blog, though it is small so that people don't nick it. I'll try the small image on a different forum, just to check igf it loads OK there.
I'd show you on my blog but I'm not sure I'm allowed to link to it.