The only ways to do it are to adjust the image size or convert it to a different format. .jpg's are to pictures as mp3's are to music, they are compressed files. Both formats are simply compressed, optimized file with minimal quality loss. That is way it is "optimal"
This was my source link, there are thousands of them out there.
Or go to htmlgoodies.com, there is everything you could ask to learn there.
None of what I read supports your assertion that file conversion and optimization are the same thing.
To optimize a file, it would have to be converted, but just converting a file wouldn't necessarily optimize it.