Cheers Artak, however that problem is ...
SOLVED! It was weird, the problem was with the picture file. I deleted it, copied it over from My Documents again and tried redoing the briefing/overview and all was well.
HOWEVER - I now have 2 new problems!
i) Where can I change the name of the mission as it appears in the SP mission list? I must have made a typo at some point `cause it's wrong, but all the files have the right name and I can't find it mispelled anywhere! I'm on about the mission name that you click on to show the overview.
ii) At the start of the mission the mission briefing has a big gap in it, which I didn't put in. It's on the second page. There's a couple of lines of text at the top and one half way down, just above the mission objectives. There's one arrow half way down the screen. Click this and it removes the mission objectives altogether but presents the text correctly. The objectives just completely disappear but the text all looks fine, no big gap in it any more.
Any and all help appreciatively received.