PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
PixBuilder - Free Photo Editing Software for PC
PhotoFlare - Quick, simple but powerful Cross Platform image editor.
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.
PhoXo - PhoXo is small and free image editing application.
Photoline - PhotoLine: Bildbearbeitung, Layout, Vektoreditor, Bachkonverter und Webbildbearbeitung