GIMP is a free program for such photo retouching, image composition and image authoring. Essentially it's the closest thing to Photoshop you can get for free.
GIMP vs darktable opinions
Both tool have totally different usage purpose. Gimp is a photo editing and digital art tool without any photo management usage.
Darktable is a photo editing tool, GIMP is a retouching tool.
It's a photo editing app, not a gallery/catalog app.
GIMP is not a very good virtual darkroom tool, but it is a good editor and compositing tool.
Create incredible photos anywhere with all-new Lightroom CC and 1 TB of cloud storage. Get powerful editing features on any device in a new, easy-to-use interface.
Adobe Lightroom vs darktable opinions
Snapseed is a photo enhancement and sharing app with a powerful suite of imaging filters and tools for anyone, anywhere to transform any image into an extraordinary photo.
Snapseed vs darktable opinions
cannot open raw camera files.
From high-precision light and color tools, to advanced clarity and dehaze settings, to the ability to create and save your own filters. Polarr is the ideal photo editor for bloggers, students, and photo professionals alike.
PhotoDirector is CyberLink’s professional level photo workflow application, extending from award-winning video editing software, PowerDirector.