Compare images side by side
K puts this image and the next one next to each other. Arrow keys move the pane you pick.
Compare mode is for the question "which one?" Two crops of the same shot, four outputs of the same prompt, an edit and its original. Put them next to each other, zoom into the same spot in all of them, and decide.
Start
Press K. You get two panes: the image you were on, and the next one in the folder. The right pane has a gold border and a small arrows mark in its label. That is the active pane.
If you were on the last image, the second pane shows the one before it instead.
Move
→ and ← move the active pane through the folder. The other pane does not change. So the common flow is: press K, then tap Right until the right pane shows the one you want next to the left.
Add more
Press A to open another pane on the current image. It becomes active, so you can arrow it to a different image straight away. Four panes at most. With three, the grid leaves one cell empty; with four, it is two by two.
You can also drag files in from Finder or from the thumbnail strip. Each one becomes a pane. The last one dropped is active.
Pick a pane
Click a pane to make it the active one. The window title and the thumbnail strip follow it, so what you see is what the arrow keys will change.
Zoom and pan together
Scroll to zoom, drag to pan. All panes move together, so you are always looking at the same region of each image. Rotate and flip apply to every pane too.
Leave
Press K again, or close panes with the X in the corner until none are left. Compare mode does not change anything on disk.