I use the export command
to create PNG images with transparency and some fixed dimension in pixels (1280x960). Strange enough, Finder (I am on macOS) shows me a "resolution" of 144 for the exported images. Indeed, I can add the Option
ImageResolution->integer in the export command and if I do so, Finder afterwards shows me the chosen integer as the "resolution". This makes no sense to me as the images have the same dimension in pixels and have the same file size. In principle, I could just ignore this issue, but unfortunately it matters when I import the pictures into Keynote. If I import a 1280x960 pixel image with a "resolution" of 144 into Keynote, its "Original size" according to Keynote is 640x480.
Edit: So apparently PNG has an optional header with the DPI
which is probably controlled with ImageResolution->integer setting. But what does Mathematica exactly do when I crank up ImageResolution from 72 (what I assume is the standard resolution)? Compute more subpixels and then average over the subpixels to create one pixel?