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To delete several pictures, it saves some tapping if you first mark the ones to delete. This is done by either a long press on a picture or, from the overview page in Gallery, tap on the 3 dots.
From my menory (at least in earlier firmwares) the "Factory Reset" is not a real factory reset in that it doesn't delete the pictures. (It only restores the 3 default graphic files that I always remove. This is on my Galaxy Watch first gen.)