Antelope is a tool for reducing the size of JPG, PNG and GIF files, without apparently affecting the images.
The program supports lossless compression, which doesn't affect image quality, or you can enable lossy compression for greater reductions in file size.
The program is easy to use - just drag and drop your images onto the Antelope window and click Optimize. The program then uses OptiPNG, JPEGTran and Gifsicle to apply lossless compression to your images, reporting any file size reductions as it works. Results vary depending on your source images, but will typically be very small, just a percentage point or two.
If you'd like to improve PNG compression then setting Antelope's "Optimization Factor" to "Max" may help, although this can significantly increase the time the process takes (a single large image might require a minute or more). If you need to compress lots of very large images then test this setting with just one, first.
Antelope also provides separate "Lossy Compression Mode" settings for all formats, and enabling this can produce spectacular reductions (our test PNG file sizes routinely fell by 60% or more). But the program is taking drastic action to achieve this - like reducing PNGs to 256 colours - and while it's doing so in an intelligent way, you may well find the loss in image quality is too great.