Timing decides whether it is watchable
Slide duration is the setting that most affects how a slideshow feels, and the instinct to keep things moving usually produces something too fast. Three to five seconds per image suits photographs with a single clear subject. Group shots, landscapes and anything with detail people will want to search need five to seven, because the eye needs time to travel around the frame.
Transitions should be much shorter than slides — roughly half a second to one second. A three-second transition between three-second slides means the viewer spends half the runtime looking at a dissolve, which reads as sluggish. Crossfades are the safe default; wipes, spins and page curls date a production immediately and draw attention to the software rather than the pictures.
Total length is the constraint people ignore. Attention drops sharply after about three minutes for a personal slideshow, which at four seconds an image is roughly 45 photographs. Editing 200 holiday photographs down to 50 is almost always the single biggest improvement available, and it is the step most often skipped.
Mixed aspect ratios and the letterbox problem
Photographs from a phone arrive in a mixture of portrait and landscape, and a slideshow has one fixed frame. Portrait images in a 16:9 frame leave large empty bars at either side — pillarboxing — which looks unfinished when it happens repeatedly.
The options are all compromises. Cropping portraits to landscape loses the top and bottom, which is destructive for a full-length portrait but often fine for a scene. A blurred, enlarged copy of the same image as a background fills the bars attractively and is the standard solution in consumer software. A slow pan down a portrait image — the Ken Burns effect — fills the frame while showing the whole picture over time.
If most of the set is portrait, consider making the whole slideshow vertical at 9:16 instead. That is the right frame for anything destined for a phone or a stories feed, and it inverts the problem rather than fighting it.
Music, rights and delivery
Music transforms a slideshow, and it is also the fastest route to having one muted or blocked. Commercial recordings are covered by two separate copyrights — the composition and the specific recording — and platform content-matching systems detect them reliably even under other audio. A slideshow posted publicly with a popular song will typically be muted, age-restricted or have its revenue redirected.
For anything shared publicly, use properly licensed production music or Creative Commons tracks with attribution terms you have actually read. For a slideshow shown at a private gathering and never uploaded, this is not a practical concern.
Cut images to the music rather than at a fixed interval where you can — changing on the beat is the difference between a sequence of pictures and something that feels edited. For delivery, export at 1080p unless the source images are genuinely high resolution, since upscaling adds file size without detail, and remember that location metadata in the source photographs is worth stripping before anything goes online.