To find the Images folder on your Mac, follow these instructions from the Installation Instructions for Graphic Mods post:
Open Finder and navigate to the game folder for Virtual Families (it is placed on the desktop by default, but you may have moved it to Applications or another location)
Open the Virtual Families folder
Create a new folder and call it something like Images-Backup
Right-click (ctrl-click) on the Virtual Families application icon and click Show Package Contents
Open the Contents folder, then the Resources folder, then open the Images folder
The file you need is bigheads00.png