Image cropping with TinyMCE in Mosaico
I implemented an image editor which you can grab here: https://github.com/lcdservices/biz.lcdservices.mosaicoimageeditor
I built it for a specific client and have not yet tested how portable it is in other installations. But at the very least it should get you started.
Actually -- just reviewing it, and the method for storing the image via API is client specific. At some point I'll generalize it so it's useful for the community. It implements: https://github.com/scaleflex/filerobot-image-editor