The trick to getting the images to size again is to make sure that you have an extra layer in your model. You can set this to unseen in the layer browser so that it won't load into layout later, but essentially, all you need in here is a box - make it the right size for your scaled images in all three dimensions, then when you reload your backdrops, if they aren't sized correctly, simply select the bounding box as your foreground layer, go into the backdrops page of the display options and then click Automatic Sizing for each of your backdrops. Job done.
