Knowing which object mouseovered from asp .net code

Knowing which object mouseovered from asp .net code

I want to change the background image of a div class depending on which object (image etc.) is mouseovered.

So, I have in my CSS class 
  1. background-image:<%= eventTypeBG %>;
This works fine if I just hard-code the  eventTypeBG variable in asp.net code behind file. 
But is there a way I can know from code behind which image/object is being mouseovered - so I can assign a different background image?
Thanks.