Insert Image:
Using Insert Image is the best option for single-image templates (there is only one photo to be placed on a page).  No correction is required with this method.

Proofs&Prints:
To easily allow dual-orientation proofs to be created, P&P automatically crops the resulting document to the size of the appropriate orientation layer. If you want to preserve the entire document, or if you want to make the resulting size of a single-image template a little bigger, you just need to add a "sizer" layer inside each orientation group that matches the desired size.

When you want to preserve the full page of an album template that only has one dual-orientation image region, you need to put what I call a "sizer" layer inside each orientation (both the horizontal and vertical). Here is the easiest way I know to do this:

1. Select the "Image" layer in the Layers palette of one of the orientation groups
2. Choose "Select > All"
3. Choose "Layer > New > Layer..." (use the menu item and not the "New Layer" icon so you change the settings)
4. In the dialog that appears, change the name to "Sizer", change the Mode to "Overlay" and check the "Fill will Overlay-neutral color (50% gray)" box
5. Click "OK"
6. Copy the Sizer layer in to the other orientation group

You won't see any visible change in the appearance of the template, but there will be a new layer in the Layers palette that corresponds to the size of the entire document. Now when P&P runs, it will compute the bounds of that orientation to be the full document because of the Sizer layer.