I am trying to achieve the following:

1. Save all kind of images it can find (cover, back, booklet, etc.) as high-resolution files
2. Additionally embedded a low-res version of only the cover as a tag

At first glance it appears that version 3 can do this now, with different settings for embedded and external files. However, the option to "Embed only a single front image" is ignored when ticked, and Picard will embed all images I want as external files also as tags (it does resize them though as I request for tagged images).