Sorry - my mistake: didn't notice this was V12 area...
The double-click thing is just a short-cut to make a rectangle the same size as the photo (for X3/X4 only I think) - just turn snap to objects on and drag from one corner to the other.
Did I say "blend"? If I did, again I was in error - "Contour" - different tool. Although if you wanted to you could use it this way: Create another rectangle the size of the area you want to be 100% visible, then blend between the outside one and this one. Same results.
The instructions might be subtly little different if you don't have an "other..." button at the bottom of the texture list (click on "edit..." then "load...") but other than that, it should work.