There is a section about actions in the FAQs: /t/967/1
Now, for the action "Export cover" (which you can also trigger as "Action (quick)"):
If you want so collect the picture files in a particular folder, use something like this as
Format string: d:\cover pictures\%album%_%_cover_type%
This produces a file like "SOS_Front.jpg" in the folder d:\cover pictures.
You have to find out whether this is good enough or whether other information is needed to make it a unique filename.
Set which cover you want to export. If you specify nothing then all types of cover will be exported. And set if you want to allow duplicates (I wouldn't).
If you leave out the "d:\cover pictures" then the picture can be found in the folder of the audio file.