JPG Compress is used to compress jpeg and optimize png files. Program supports lossless optimization, which is based on optimizing the Huffman tables. And so called "lossy" optimization where in addition to optimizing Huffman tables user can specify upper limit for JPG/PNG quality.
It's also support lossy compress. The quality setting will reduce quality of those source files that were saved using higher quality setting. While files that already have lower quality setting will be compressed using the lossless optimization method.