Like in the title. Method: double click on the image; from the context menu on it; or any else.
In fact, it may be done with a saved action (...group) + a tool, but it is not the most comfortable solution: export the cover(s) by filename (+type: "01. ABBA - The day, before you came_____Front.jpg") into the win temp folder (CCleaner clears it periodically), then open it with a tool. Sorry, 2 steps, 4-5 clicks, instead of doubleclicking on the image. (Or, the better: from the right click menu of the record in the table.)
My actual workaround: foobar can show it. Not the most comfortable way, too: many clicks, no zoom. Or: export all images to their folder, then delete them after using.
By the way (as a second suggestion), it will be nice, too, to have a new action type: start an external application with parameters. As tools do it. (Or start a ready tool -- similar, as we can already use the ready exports in actions.) So, for example, will be easy to create simply an action group for the above task.