Theme and Multiple Dialog

Theme and Multiple Dialog

Hi everyone !
I have a problem with theme with my ui dialogs.
I have 2 dialogs on my page and I what for them a different background image.
Each dialog is in a different ASP webpart.
so i put on each webpart 's css file: .ui-dialog { background:url(myImage;}
The problem is all the dialogs have the same background: the first I defined.

How to have two different background for my dialogs?