If you are wanting to change the BG on a multi page cue, such as a Lyric, drop the media file on the "Cue" and not a "Page".
If you are using multiple media files in a multi page cue, unfortunately, there will be no way to add this ability. There can be only one BG applied to a Cue. The other media files are added as "Objects" which sit on top of the BG.
However, if you make "Full Stretch" your default "Preset", when using multiple images in a multi page cue, they will be full stretch automatically when you drag them in. So once you apply the Full Stretch Preset, click the Apply Preset button again and select "Set As Default".
Let me know if I am missing something.