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 Affinity Photo opinions
Free and powerful, but complicated, wich makes a difficult workflow
It doesn't have non-destructive adjustment layers.
Krita is a professional Free and open source (FOSS) painting program. It is made by artists that want to see affordable art tools for everyone within concept art, texture and matte painters, illustrations and comics.
Krita vs Affinity Photo opinions
You draw, you don't edidit photos
The text tool is awful. Not even close to Affinity.
Glimpse is an open source image editor based on the GNU Image Manipulation Program. The goal is to experiment with new ideas and expand the use of free software.Discontinued
The Glimpse editor has been discontinuedSource: https://glimpse-editor.org/posts/a-project-on-hiatus/
Graphite is an open source, cross-platform, 2D vector and raster general-purpose graphics editor with AI image generation features and a non-destructive node-based effects compositing engine.
Hugin is an easy to use cross-platform panoramic imaging toolchain based on Panorama Tools. With Hugin you can assemble a mosaic of photographs into a complete impressive panorama, stitch any series of overlapping pictures and much more.i
Photoflare is a cross-platform image editor with an aim to balance between powerful features and a very friendly graphical user interface. It suits a wide variety of different tasks and users who value a more nimble workflow.
Pixelitor is a cross-platform raster graphics editor written in Java. It supports image filters, drawing, layers, multiple undo, etc. Currently its main strength lies in the more than 70 image filters and color adjustments, some of which are unique.
Seashore is an open source image editor for Mac OS. It features gradients, textures and anti-aliasing for both text and brush strokes. It supports multiple layers and alpha channel editing. It is based around the GIMPs technology and uses the same native file format.
gpaint (aka GNU Paint), a small-scale painting program for GNOME, the GNU Desktop Environment. gpaint does not attempt to compete with GIMP, it is just a simple drawing package based on xpaint, along the lines of Microsoft Paint.