It would be used as default one instead of Pillow, to process JPGs and PNGs (it could also be in front of Pillow and take care of the resizing, we'll see).

For JPGs Pillow's performance seems abysmal compared to IM used with the right options (10x slower): P2540

The community-maintained GM engine might be a source of inspiration: https://github.com/thumbor/graphicsmagick-engine

It's actually quite surprising that there's no IM engine for Thumbor. Probably because the performance boost provided by jpeg:size is so little known (it's absent from all IM python bindings, I had to monkey-patch it in).