I'm trying to do the same thing, the picture i have put for PlacesListExtended is showing only the portion within the sizing margins which can be stretched so i tried to set defaultpanelsize. However this property was not there in any of the fields so i added the property to each field. and set the defaultpanelsize to the size of each pic. However when i try to test it an error pops up saying "You may have added or edited a property incorrectly". I tried to change the defaultpanelsize however the error still came up.

I've tried to add the content margins to each field and change that too but i'm still getting the same error. I just don't understand whats happening here.

Edit: I found out how to get around that error - copy the property from a vista theme opened directly in Vista Style Builder and paste it into the windows 7 theme in the windows 7 style builder. I don't know if there's a fix for this but this method works.