App Store Screenshot Design Tips: 14 Practical Rules

The short answer

Good store screenshots are read, not looked at: one benefit per screenshot, a caption readable at thumbnail size, the strongest reason first, and one visual system across the whole set. Everything else (frames, colors, 3D phones) serves those four.

Make it readable at a glance

Lead with the benefit. “Never miss a bill” tells people why they'd install; “Reminders” only names a feature.
Design for the thumbnail. In search results a screenshot is a few centimetres tall on a phone. Two to six words, large and heavy, fit; a paragraph doesn't.
One idea per screenshot. If a caption needs “and”, it's probably two screenshots.
Contrast wins over decoration. Dark text on a light background or the reverse; avoid text over busy photos unless you darken them.

Tell a story in order

Put the strongest reason first. The first three screenshots are what most people judge you on; don't spend them on onboarding or settings.
Show the app doing its job. The main screen with real-looking content says more than a feature list.
Zoom into what matters. When the interesting part of a screen is small, enlarge it next to the phone so it reads at thumbnail size.
End with reassurance. Later screenshots suit privacy, sync, widgets or awards: things people check after they're already interested.

Keep the set consistent

One type family, one caption position, one color system. Variation inside the system (alternating backgrounds, a phone that moves) keeps it lively without looking like five designers worked on it.
Mind continuous designs. Backgrounds that flow from one screenshot into the next look great on the product page, but search results show screenshots one by one, so each must still work alone.
Localize, don't just translate. Captions in German or Vietnamese often run longer than English ones; check every language for wrapping and shorten where needed, and use screenshots of the localized app.

Keep it honest, then test it

Only claim what's true. Ratings, awards and “#1” claims need to be real and current; both stores reject misleading metadata, and Google Play's policies discourage ranking claims in listing graphics.
Match the real app. The interface in your screenshots should be the one people download.
Test, don't guess. App Store Connect's Product Page Optimization and Google Play's store listing experiments let you show different screenshots to part of your visitors and compare installs.

A quick check before you upload: shrink the set to the size of a search result on your phone. If you can't tell what the app does from the first three screenshots in three seconds, rewrite the captions before you redesign anything.

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Questions

Should screenshots use a device frame?: It's a style choice. Frames make it obvious that people are looking at an app; frameless, rounded screens give more room to the interface. Either works if the whole set uses the same one.
How long should a screenshot caption be?: Short enough to read in a thumbnail: usually two to six words for the headline. A subtitle can add detail, but the headline should make sense alone.
Light or dark screenshots?: Whichever makes your interface and captions clearest, and stands out from the apps people see around yours. Many sets alternate the two to separate screenshots.