Selected manifest: https://iiif.bodleian.ox.ac.uk/iiif/manifest/15812b3d-2eb5-40be-a806-7d66e629fe39.json.

The image cropper included here is a tool developed by the Bavarian State Library. A few comments about its usage may be in order here.

(a) You can activate it by clicking the icon that looks like from the menu on the right.
(b) It is designed to work with single view only, not book view (facing pages). You may have to switch to single view.
(c) If you go into full-screen mode and select a region to be cropped, it may seem as if clicking the share icon does not work. The issue here is that the modal to be triggered by clicking the icon is a tad shy and is hiding underneath: exit full-screen mode to view the modal, configure your image crop, etc.
(d) It is recommended that you activate the cropper first, resize the selectable region a bit and zoom in afterwards. You may experience some issues if you zoom in directly and activate the cropper at a deeper zoom level.

Need more information about using Mirador? Pádraic Moran (NUIG Galway) has created a very useful walkthrough video on YouTube, which he created for MIRA, and of course, Mirador has a website.