[jrwoodruff]

Photoshop's save for web option makes quick work of these decisions, if you have access to a copy. In the '4-up' view you can visually compare your original file to three different formats with information on file size and precise control over color indexing, etc.

The save for web option will also strip out any extraneous info, such as exif data, that can add to the image size.

[xsmasher]

Agreed, Photoshop is the right tool for the job here, not a command-line tool. Some images will need more colors than others, some will need more dithering than others; and the tool used here doesn't look like it picks palettes very well.