Every slide deck has a fixed frame, and almost every pasted image is bigger than it. Widescreen decks are 16:9, so 1920 x 1080 px is the common full-slide size; older 4:3 decks sit around 1600 x 1200 px; and a deck authored for a Retina display wants double the logical size, which is 3840 x 2160 px for a 1080p slide. The rest is arithmetic. A 6000 px camera photo pasted into a 1920 px frame is storing more than nine times the pixels the projector can resolve, and the deck file pays for all of them. Resizing down fixes that with no visible change on the screen — you are removing data that was never going to be displayed. Resizing up does not work the same way: stretching a 1000 px image to fill a 1920 px slide adds pixels and no detail, and it looks soft where it counts. So resize down to the frame, and only go to the Retina target when the deck will actually be shown on a Retina screen rather than exported to PDF or sent. Skwish resizes to exact dimensions and compresses in the same pass, in the browser.