When I multi-select snapshots, I expect to be able to change the background image and have it apply to all selected snapshots. Instead, I have to update the background image on each snapshot one by one.
This can be done by setting which snapshots are affected for the changes you make (Scope edit). This is not only for background but for any changes. Right click on a snapshot in the storyboard browser and find the option "Set affected snapshots" . THis gives you all options of how the changes that are made are propagated to other snapshots.