Photoshop 6 - can be made portable using a launcher (script here)
IrfanView Portable - IrfanView image editor/thumbnail viewer packaged for portable use.
XnView Portable - XnView is a freeware image browser similar to ACDSee.
GIMP Portable - Portable version of GIMP photo and image editor
Art Icons Pro - can be made portable using a launcher (script here)