RIOT (Radical Image Optimization Tool) presents a before-and-after image compression view that allows you to reduce file size of an image into JPG, PNG, or GIF format. Includes optional metadata removal, batch operations, compression to a target file size, transparency options, and other common editing functions (flip, rotate, zoom, pan). Notable versus similar software in the ability to gradually reduce the available colors one-by-one (7, 8, 9, 10…), not just milestones of 16, 32, 64 etc.
The program is also available as a plugin for XnView, GIMP, or Irfanview (see website).
Functional in Linux using Wine. Many of the official entries here on PortableFreeware have used RIOT for compression. Note that the installer version of this program is considered adware.