Coulr
Coulr is a free and open source small color selection program for Linux
Pixelitor
Pixelitor is a free and open source, multi-platform advanced image editor with support for layers, filters, cropping and other functions
Curtail
Curtail is a free and open source, simple and useful software for compressing images in Linux
ImEditor
ImEditor is a free and open source, simple and versatile image editing program
KColorChooser
KColorChooser is a free and open source small and simple program for mixing colors and creating custom color palettes
ColorMate
ColorMate is a free and open source small app for converting color between three notations #rgb, rgb() and hsl() for Linux
Picturama
Picturama – free and open source, multiplatform software for organizing and viewing images
Photoflare
Photoflare – free and open source, fast and simple, but powerful cross-platform image editor
LazPaint
LazPaint – free and open source, cross-platform light editor of images with multiple tools and filters
LightZone
LightZone – free and open source, multiplatform digital darkroom software and professional-level photo editor