Fantastic free app that does exactly what it's supposed to. I need to keep multiple images on screen at once daily, this app helps me organise my screen estate much better than preview, since all images open frameless, they can even float in front of every other app in use.
Another great feature is the ability to save your opened images in a "set", so you can save and load presets for whatever project you're working on.
My only gripe with it is that you can't resize or rotate images using the mouse. It works through hotkeys but it would be much more convenient to through an interactive "hot-corner" on each image.