Antelope is a tool for reducing the size of JPG and PNG 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.
By default the program will overwrite your source files, so be extremely careful. Ideally, always work with copies. Otherwise, open the Options dialog and check "Backup original file".
Once you understand that key detail, basic operation is easy - just drag and drop your images onto the Antelope window and click Compress. The program then uses OptiPNG and JPEGTran 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, though beware: it also significantly increases 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 Mode" settings for both PNG and JPGs, 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.
One additional problem is that there's no setting to define how lossy your JPEG compression will be, unfortunately. This isn't a massive issue - the default isn't too drastic - but it's still something the user should be able to decide.