Radical Image Optimization Tool (RIOT for short) is a free image optimizer that will let you to visually adjust compression parameters while keeping minimum filesize.
It uses with a side by side (dual view) or single view interface to compare the original with the optimized image in real time and instantly see the resulting file size.
It is lightweight, fast and simple to use, yet powerful for advanced users. You will be able to control compression, number of colors, metadata settings and much more, and select image format (JPG, GIF or PNG) for your output file.