For GIMP users this may be a good news:
I am working on a GIMP plugin that offers the same functionality as the IrfanView plugin.
GIMP users will have a new option in File->Save for web with RIOT which opens the current image in RIOT for filesize optimization.
The plugin is almost done. I plan to release it in a month if everything goes well.
In the open source spirit, the source code for the plugin itself will be freely available.