Ok I think the best thing for me to do will be to upload slightly larger images to make up for the cropping, so they can display properly in firefox and just show the extra margins in opera, can you tell me exactly by how much they are getting cropped on the top and left sides so I can make up for this?
I was always intending to optimize the background image and will probably end up changing it altogether but thanks anyway for the heads up on how big it really is!