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.
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.
RealWorld Photos is an free portable photo editor for Windows. Retouch photographs and batch process images in an easy-to-use editor. Perform lossless retouching, watermarking and other operations on .jpg images.
WidsMob Retoucher is an excellent photo retouching program, which is able to reduce image noise, portrait retoucher, film pack, adjust effects, fish eye effects, vignetting and more other features to photos with ease.