Sigh, problem solved. Anybody got a gun?
So not sure why it was a problem, cause I've done the same thing before. But for my on and off screen activation tabs, those I use to slide the left dock onto and off screen, I'd made a 3rd party app button and used a 73x73 transparency. Then stretched to the size I wanted it. Like I said, I've done this before and maybe just by chance haven't resized it too large.
But I'm thinking there must be some kind of internal limit as to how far the image can be stretched? Whatever, as soon as I replaced the old transparency for a 170x300 one, which is the size of the tab I was using, it exported from themebuilder fine.
Ah well....lesson learned then.