Must be in the Display Resolution (if the file is anamorphic). See the Anamorphic Video section for details.
Must NOT have subtitles visible, unless they are hardcoded.
For files with hardcoded subtitles, try to include at least one screenshot showing the hardsub. This goes for both cases: whether the file has hardcoded subtitles for the whole movie, or for only the foreign language parts.
It is highly recommended to optimize your PNGs to reduce their file size with zero loss in quality. See the Lossless PNG Optimization section for details.
Avoid taking screenshots of timestamps too close to each other.
Avoid putting too many screenshots. For example, a 15-minute short film does not need 10 screenshots.
Avoid scenes where nothing much is visible in the frame — for example an almost black frame or an extreme closeup of an object.
For multi-file uploads, take at least one screenshot from each file.
For full disc uploads, try to include screenshots of the disc menu — in particular for the Extras or Audio / Subtitle options. See the Disc Menus section for details. (placeholder link)
For HDR or 10-bit video, keep the bit-depth of the PNGs as 8-bit instead of 16-bit. The Programs Choices page covers ways to set up your program to use 8-bit PNG. Also see the Lossless PNG Optimization section for how to convert existing 16-bit PNGs to 8-bit.
For silent movies, try to include at least one screenshot showing the intertitles, especially if they are not in English.