Graphic Assault System 128 v3 by Bruce Bowden (released September 6, 1988) is a graphics viewer / converter for the Commodore 128.  Supports Koala, Doodle, Old Gas format and Hires images. 

Included in the archive is a text file with the keyboard commands.  

GDS 128 by Eric C. Rafanan (1990) is a utility for display and converting GIF image files.  Not only can it downsample colors, it can also save out the image in a format that is compatible with BASIC 8.  Full documentation is included in the archive on the disk image.

I-Port by Rick Kane is a graphics importer / converter for I-Paint 128.  You can also export files to GIF format.