Background images will not show on ipad 
Автор: Donald M.
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That basically sums up the issue. Apple Help is no help at all.
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OK - I changed the attribute for these images from "Do not repeat" to "Fit to Window" and now they display,
but
now I only see a tiny part of the image, like it's been zoomed in by a ridiculous amount.
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Managed to fix the problem although it's more of a workaround.
The answer is that you can't use Fixed Parallax, you have to use Parallax. Then it doesn't matter which of the options I mentioned above you use.
IPads are weird.