Since Phiewer version 1.5.2 deleting files is possible.
You can quick-delete a currently selected image with a menu command or two keyboard shortcuts. These keyboard shortcuts are:
A currently selected file gets deleted immediately with no confirmation dialog. The file gets not deleted “hard” on the filesystem, it will be moved to the systems trash like every file deleting command works on your macOS.
At the bottom of the thumbnails container the delete button is also available for deleting the currently selected image. Hitting this button accidentally will not have a non reversable effect. A confirmation dialog is showing up first to ask you if you really want to delete this file.
Phiewer – Mac os image viewer