Close window on ESC
Similar to eye of gnome, the window (or perhaps tab if #161 (closed) becomes a thing) should be closed by pressing escape, as this makes for a very quick way of inspecting different images in disparate locations. Many other applications use escape as a way of exiting out of a dialogue, and Loupe sort of acts as a dialogue of whatever program wanted to view an image.
My personal use case is that I have an extra button on my mouse that I have bound to escape that makes this extra handy, but I feel it would be generally useful regardless.