Comment to Mobile browser vs. PC browser clarity
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what do you have the background set to, scale, auto, fixed.. ? keep in mind it has to work with a bitmap image, as a dirty hack a larger size image will fix this set to auto, just make sure its not a super big file so you aren't penalizing your users on mobile waiting for this huge file to download..
you can also pre-cache a specific image, but that is a whole other story entirely.