Duncan is right to recommend “2x” (144ppi) at the largest pic size you will display. Three years ago I tested retina graphics extensively and found that 72ppi images look blurry and awful on retina iOS devices and Macs, but 144ppi looks very good, and 288ppi is great but almost too sharp. So 144ppi is the ideal in terms of look and filesize.
I usually can compress 144ppi JPGs to 44% in Photoshop without really seeing compression artifacts when displayed on a retina screen. But if there is a lot of tiny red text, I may need to take it up to a 50% compression to make it look nice. And when I need transparency, I tend to use PNG more than GIF because the filesize is smaller, and I use TinyPNG to compress my 24-bit PNGs into 8-bit files without noticeable loss in visual information.
So it seems that Sparkle will use your original pic as is, like a Freeway pass-thru image, when Sparkle displays that image at it’s actual 144ppi size, but Sparkle (or rather, the browser) will shrink the image when that same pic scales down on a responsive page, and Sparkle will present a lower rez 72ppi version of the pic when Sparkle wants to display it on non-retina machines. But how fuzzy or sharp that 72ppi image will be is not something I’ve tested in Sparkle, nor do I know if 8-bit PNGs will remain 8-bit PNGs (and so on) when Sparkle does it’s image manipulations. What I find in Photoshop is that if I make a 144ppi image to be 72ppi, it needs a little sharpening to look its best, unless I use the “sharper” resize antialias setting in Photoshop. So an extensive test in Sparkle would be necessary for one to know exactly graphics are output to both 72ppi and retina devices.
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