Huh, so there is! It even supports batch converting if you select a bunch of files at once and click "Convert Image" which is awesome!
For OP, I'd check this with the JPGs from your camera to see if it's worth the effort. I just tried it on a few JPGs I have and some of them it only saved ~20% (from 6MB to 5MB) whereas larger ones it saved 75% of the space (from 60MB to 15MB)! So it probably depends a lot on the settings your camera uses for saving JPGs.