If you like fractals you could fire up Apophysis and make one of your own! 4320x1920 would be all youd need. Or use google image search and set the size to larger than 12mp. Just make sure that its dimentions are no smaller than 4320x1920 or 1920x4320. Then you something (paint, etc) to cut it down to a rough shape.
Yea, I like abstract stuff, but I think the best bet for me is to go simple minimalistic graphics (maybe vector) on a solid color background, and just pick 3 random ones, then photochop it to fit the portrait screens.
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