This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Previously we had no idea what algorithm generated thumbnails, this starts tracking the version.
We also bumped up the version to force all optimized images to be generated. This is important cause we recently introduced pngquant which results in much smaller images.