Currently, it is not directly possible to add an icon or image to the tabs in tab containers. Instead, there are ideas such as creating css classes. Also with Mendix 10, creation an icon collection feature is coming so that they can be used as css classes. see: icon collection
It is also possible to use enumaration values as strings in the 'show badge’ part in the tabs. I think it should be also possible to be able to insert enumaration images to the 'show badge’ part or tabs so that they can be used as tab icons.