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 Corel PaintShop Pro opinions
Just as good - and simpler
ive used them both and i like gimp
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/
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.
CinePaint is a deep paint application that edits EXR, DPX, 16-bit TIFF, JPEG, PNG and many other types of image files. CinePaint is available as a native application for Linux, Mac OS X, and BSD.Discontinued
The development is discountinued since 2013-05-06https://sourceforge.net/p/cinepaint/activity/?page=0&limit=100#599c65d15fcbc926952bcea9