Default Pull down and framework background info???

Hello everyone, I have recently been attempting to do some minor theming and I can't seem to understand the sizing of the images. I was told it was supposed to be 7201280 but with some images it just doesn't seem to work. I've also noticed with some zips I've downloaded that sizes were different then the one I mentioned. Can someone please explain to me the proper sizing of each?? Thanks in advance!

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