Gallery view "Picture" cover like in desktop app

[wtfzambo]

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• [X] I have read CONTRIBUTING.md
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Clear and concise description of the problem

The Android app only has the "Page cover" option when selecting a cover for Gallery view.

Suggested solution

Add the "Picture" option like in the desktop app (see screenshot).

Alternative

No response

Additional context

Having the "Picture" option makes creating an easily browsable catalog of bookmarks extremely seamless. On desktop, this is already possible (see example screenshot again):

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I'd create a PR but know nothing of Android development D: