Pacing and the Ken Burns effect
Three to five seconds per photo suits most slideshows: long enough to take in a face, short enough to keep moving. Photos with detail or text want longer; a run of similar shots wants shorter. Slow motion across a still image — a gentle push in or drift across the frame — keeps the video feeling alive instead of like a series of freezes, which matters because a completely static frame is what makes viewers scroll away. Keep the movement subtle; fast pans on stills look like a screensaver.