This looks amazing. Will spend more time with it but the first impression is really good. However, I think that the compression could be triggered by a user instead of recalculating on values change. For example, my attempt to change dithering and colour palette slows down the responsiveness of the page a lot for me. I can also see a download button so I assume I can try using it instead of http://optimizilla.com/ with which I have been more than satisfied till now but it does not have as many options as I can find in your app.