Samsung
Change Album Cover Image auf Samsung
By default, Samsung Gallery treats every album cover as dynamic. Each time you add a new photo or video, the app replaces the thumbnail with that latest file, which is convenient for albums you never touch again but frustrating for anything you keep updating.
To lock in a specific cover, open Gallery ➔ Albums, then tap and hold the album you want to change. Tap More, followed by Change cover image. Choose any photo or image from that album, adjust the crop as needed, and tap Apply to confirm.
What actually changes
Once you apply a custom cover, Gallery stops updating the thumbnail automatically for that album. New photos still get added to the album itself, only the cover tile stays fixed.
This setting lives per album, so you need to repeat the steps for each album where you want a fixed cover. There is no global toggle that applies the behavior everywhere at once.
If an album cover keeps looking wrong after you have set one, double check that you edited the correct album, since Samsung Gallery sometimes lists similarly named albums (device folder vs. synced folder) separately.