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
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
RawTherapee
RawTherapee is a free and open source, cross-platform raw image processing program
Darktable
Darktable – free and open source cross-platform software for processing and cataloging RAW images
XnConvert
XnConvert is a fast, powerful and free cross-platform batch image converter, It allows to automate editing for your photos
Nomacs
Nomacs – free and open source multiple platforms fast image viewer with additional editing functionality