I've discovered that it is pretty easy to do it manually. I simply grab an image in the center and align it with the left margin. Next, I grab an image in the middle of the right border and drag it to the left. Then I select Frame Fitting > Fit Content Proportionally. This leaves the top and bottom margins of the frame quite a ways from the image, so I simply drag them into position.
This works pretty well, except it's sometimes hard to get an image the correct size to span the page from one margin to the other.