RIOT or Radical Image Optimization Tool is an image optimizer which performs different modifications on an image such as the number of colors, the type of compression used and the format in which the program has been saved. It is a freeware image compression application that's well-designed.
Here are some screenshots to illustrate the features and user interface of this application:
Converseen
JBatch It!
XnConvert
Adobe DNG Converter
Nconvert