Ok, I've done EVERYTHING as outlined above (twice) and it still isn't working...not even the colours are changing onscreen let alone a picture I've selected coming up....and now I've gone back in to the design my profile screen and all those colours and transparencies I've included aren't showing at all (yes, I did save!!).
Also what is the difference between 'background image' at the top of 'design my profile screen' and the CSS instruction?