Recent iPhone sizes can't go to Google Play as they are
Google Play doesn't allow the long side to exceed twice the short side. iPhone Dynamic Island sizes (1179×2556, 1206×2622, and so on) are just over 2:1, so they're rejected as is. Export the same screen at 9:16 (such as 1080×1920) for Play. The Home Button iPhone size (1242×2208) is 9:16 and also meets Play's phone recommendation.
Neither store accepts an alpha channel
App Store Connect doesn't accept images with alpha channels or transparency, and Google Play takes screenshots and feature graphics as 24-bit PNG without alpha or JPEG. A transparent background option in your design tool's PNG export adds alpha. This tool detects alpha from the PNG header's color type and any transparency (tRNS) chunk.
FAQ
How many App Store screenshot sizes do I need?
For an iPhone app, one set for the Dynamic Island medium display (such as 1206×2622) is required. If you support iPad, one set for the 13-inch display (such as 2064×2752) too. Other display sizes are scaled down by App Store Connect if you don't upload them.
Can I upload only the largest iPhone size?
The required size in Apple's docs is the Dynamic Island medium display. If you upload only a larger size (such as 1320×2868), check in App Store Connect how the medium slot is filled. This tool marks the rows the docs list as required.
What is the recommended Google Play screenshot size?
Play Help says phone apps with at least four 9:16 portrait screenshots of 1080×1920 or larger (or 16:9 landscape of 1920×1080 or larger) become eligible for large-format promotion. The basic rules are a short side of 320 px or more, a long side of 3840 px or less, and within 2:1.
Is the image I pick uploaded?
No. Your browser opens the image to measure its width and height and reads only the PNG header info at the start of the file. Nothing is sent anywhere. Copy link puts only the width and height in the URL.
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