In android, when you're in gallery, or photo app, you can click the weird share logo, and it let's you create a new post with the photo, or video from your gallery, etc.
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I’ll post a vid in a few minutes.
Here's how to do it on Amethyst on Android.
https://video.nostr.build/6486f8c8b08c906f71c41085fbcbd79ce19a90f715429c4232361d5335d5e23a.mp4