I understand what u want, you can achieve that using shapes option present in marks shelf.
All you need to do it,
1) go to C:\Users\<your pc user>\Documents\My Tableau Repository\Shapes and create on folder(say region) and paste all images which you want to display in your dashboard.
2) go to marks and make automatic to shape
3) drag your field to shapes(i drag region in attached sample workbook)
4) click on shapes-> edit shapes
5) select region under select shape palette.
6) assign image which you inserted as per your requirement.
And your are done.
hope this will help you.
Book1.twbx 1.3 MB
Thanks Anil, for quick reply,
i'll check and see if it solves my problem.