Disclosure: LeptonGUI is a tool I've developed. That makes it difficult for me to be unbiased, and so I'll tell you what the program does, but not try to assess how well it does it.
Lepton is an open-sourced Dropbox technology for the lossless compression of JPEGs.
The program can compress JPG files by 20% or more, exporting the results in a LEP format (picture.jpg > picture.lep). Point the program at LEP files later and it restores the original JPG file, byte for byte.
The standard Lepton.exe runs only from the command line, and can behave strangely if you don't run it in just the right way. Experienced users won't have any trouble with it, but we've also put together a very basic front end that might be easier to use.
Operations are much like any other conversion tool. Drag and drop your source files, JPG or LEP, or click Open and choose whatever you need, and click Convert to process them (JPGs become LEP, LEPs are restored to JPG).
The program won't overwrite the destination file. This is intentional, as I don't completely trust Lepton to work in all circumstances, and I don't want there to be any risk of losing data.
When you're done, LeptonGUI displays the individual savings for each file and the overall compression rate.