Note: This applies to 1.7.1 of Publisher, Designer, and Photo, even though I'm only posting in the Designer Bugs forum. It also happens in 1.7.2.414 beta of Designer and Photo.
According to the Help, the "Insert Behind the Selection" button on the Toolbar should behave like this:
The Help also tells us (in Targeting Objects):
That is true in general.
However, for one specific case it is not true. If you drag a file from the File Explorer onto the Designer document, rather than going below the currently selected object, it goes at the bottom of the entire layer stack.
I'm not sure if this is an error in the Help, or an error in the program functionality.
Recipe:
Create a document.
Create two objects of any kind.
Select the top object (second one created, or by selecting the top layer in the Layers panel).
Click the "Insert behind the selection" button on the Toolbar.
Drag an image file onto the document from File Explorer.
Note that it appears at the bottom of the layer stack, rather than between the two objects as expected from the description of the button's function.