If it's a simple combo of photos... Should be able to do it right in Google photos under the utilities tab.
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Thanks for the hint. I haven't used Google Photos in a while and its features sound like what I want: https://support.google.com/photos/answer/12637115?hl=en&co=GENIE.Platform%3DAndroid
However, it is not an open source app.
Boo hoo. Get off your high horse. It works.
The best workaround I found so far is to add a new tag to some images in my gallery app (Aves Libre), then filter for that tag and take a screenshot of it. It is neither possible to order the images nor to scale some of them. But it is definitely better than nothing ;)