If you want to remove all embedded pictures,
simply load all files,
Press Alt-T to open the extended tags dialogue,
click on the red x next to the picture symbol
click OK.
TheID3 standard suggests a field called LINK that can be used to save links.
http://id3.org/id3v2.3.0#Linked_information
I do not think that MP3tag supports this field.
The ID3 standard actually recommends to use this field only for read only storages so that the link cannot be broken ...
THere is also a special MIME type that requires a URL: " There is the possibility to put only a link to the image file by using the 'MIME type' "-->" and having a complete URL instead of picture data. The use of linked files should however be used sparingly since there is the risk of separation of files."
I do not think that MP3tag supports this MIME type.
Yes, that is the purpose of actions.
See the help on actions: Actions – Mp3tag Documentation
See also the FAQs on actions:
Actions and batch-operations