One way to get the image in the rollover is to use the text box instead of balloon. Tthe following options from the settings file might be useful for your needs:

<text_box>   <!--  text box is displayed when user clicks on an object and the object contains description. You can disable text box for individual objects in data file -->
     <enabled />
     <show_on_hover />  <!-- text box can be showed on roll over too - set this parameter to "true"  -->
     <hide_on_roll_out />  <!-- if text box is showed on roll over, it can be hidden on roll out  -->
</text_box>