What do I need to install/change so that when I get a call the picture of the caller is in fullscreen mode instead of the regular small image.
I think the dimensions of different parts of the GUI require changes in the framework-res.apk files in the internal system partition. This requires you to be rooted, of course. I haven't seen any custom themes for custom ROMs that have a full screen portrait however.
Maybe you need something like that:
Notice: Both are free just for first five contacts. For more, you must purchase full version. Both require to assign HD photos manually (from SD Card, Camera or Facebook). None of them support images stored with your contacts on your Google account.