Not sure if this fits your category but here are a few FOSS apps I have:
SuperImage for upscaling locally using your phone’s GPU;
Image Toolbox to convert, crop, resize, do background removal (by hand though, so nothing fancy like Magic Eraser), apply a bunch of filters, draw on a sketchbook, encrypt any(!) file, edit/remove EXIF data, pick colors, compare two images with a slider, generate a palette from an image;
Squawker to follow (without an account) multiple Twitter/X accounts as if they were RSS feeds with the ability to group them (cool when you’re trading and need a bunch of news sources grouped by sector, but it depends on Elon not messing around with the Twitter layout again as otherwise Squawker will also need to be updated);