If you are using Alert.show() you cannot add images in the content area. You can extend the Alert class to accept that but it's far to difficult.
The easy solution is to use the PopUpManager as that allows you to have any kind of content you want.
To do this:
1. Import the PopUpManager Class: import mx.managers.PopUpManager;
2. Create a custom component based on the TitleWindow component.
3. Assign a variable to the custom component class.
4. Call the PopUpManager Class add function: PopUpManager.addPopUp(myWindow,this,true);