As of DPL 4.1.2 on Mac, the keyboard shortcut for going to the next or previous image are the respective arrow keys. They work, but not in every case: If focus is on a field that allows entries, the arrow keys move the cursor position within that entry field.
Please add a keyboard shortcut that selects the next or previous image unconditionally, no matter where the focus lies within DPL. Most other apps I use do this with command-arrow.
One place this happens is if I selected and applied a preset. I use presets for things like “Vignette -5” which make a single setting change to preset values that I like. These are often applied last and I’m ready to move on to the next image but have to click in somewhere else, usually dead space around the photo, before I can advance.