In my 1.5.0 admin, the selection Admin->Catalog->Product Types->Product General->Edit Layout->Show Additional Images shows:
Please make any necessary changes
Show Product Additional Images...
Type: Posts; User: lat9
In my 1.5.0 admin, the selection Admin->Catalog->Product Types->Product General->Edit Layout->Show Additional Images shows:
Please make any necessary changes
Show Product Additional Images...
This field should be set to 1, not true, for the additional images to be displayed. Your naming convention is just fine.