new and need help - adding more photos to gallery photo object

Sorry if this has been asked elsewhere, I am new here and learning to navigate the forum. I need to know how to apply mouse over effects to additional photos. The Gallery > Photo Objects > any photo object that has a number of smaller images and 1 larger image, when moused over the large image is replaced by the smaller. Default has only 9 images I need 12 so I wish to add another row.

Problem : When I ungroup the objects then regroup the mouse over effect is lost. I have tried inspecting the names of the objects, and applying to new row, I have looked at the layers and the mouse over action. I am lost after that and can't find anything in the help menu or on tutorials videos on youtube that deals specifically with this. Any help is greatly appreciated.