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 PicMonkey opinions
Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format.
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/
XGimp is an adaptation for iPhone and iPad of the app GIMP (GNU Image Manipulation), which is a freely distributed program for tasks as photo retouching, image composition and image authoringThis app doesn't have any features at the moment.
PhoXo is small and free image editing application. It's tiny, fast, easy to use, can be treated as a mini PhotoShop. Its features include layers, undo, over 50 special effects, batch processing, and a wide variety of useful and powerful tools for selecting, crop, painting...This app doesn't have any features at the moment.Discontinued
No updates since 2019, and isn't fully Open Source (only the kernel is).