I don't quite understand what the window.devicePixelRatio value is, and how it dictates what size image (2x, 3x, etc) I need for that device.
For instance, on an iMac 5K Retina (Late 2015), I'd expect the pixel ratio to be at least 3 or so, but it's actually 2, the same as an iPad Air and iPhone 6s. Does that mean it prefers a 2x bitmap? 3x?
devicePixelRatio is the ratio between physical pixels and device-independent pixels (dips) on a given device. You can think of dips as what the display "acts" like.
For example: a non-retina 27" iMac has a width of 2560 physical pixels. Everything is displayed 1:1, so it's also 2560 dips wide, so the devicePixelRatio is 1.
On your retina 27" iMac, the width is 5120 physical pixels. But the display "acts" like it's only 2560 pixels wide, so that everything is shown at the same physical size as the non-retina iMac. Therefore, it's still 2560 dips wide, so the devicePixelRatio is 2 (5120 / 2560), and you would serve 2x images.
(You can look up what the dips values are for your system – if you have a retina display – by going to System Preferences > Displays > Display and switching the Resolution toggle to Scaled, then hovering over the different options. For Default, on the 5K iMac, it'll say "Looks like 2560 x 1440").