RawTherapee is a free, cross-platform raw image processing program
GottenGeography
An easy to use geotagging application for the GNOME desktop, which aims to have a very streamlined and intuitive interface that makes it easy to apply geotags to photographs.
Darktable
darktable is an open source photography workflow application and raw developer
GThumb
gThumb is an image viewer and browser for the GNOME Desktop. It also includes an importer tool for transferring photos from cameras.
Surface
Pinta is a free, open source program for drawing and image editing.
DigiKam
Professional Photo Management with the Power of Open Source
Shotwell
Shotwell is a personal photo manager.
Photo Collage
PhotoCollage allows you to create photo collage posters.
niankohoka
Krita is a FREE and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry.
GIMP
GIMP is a cross-platform image editor.