I have had this issue myself. I have no screen shots, but what I can describe is as follows: I placed several images (boxes, crates, spiral staircase and book shelves) on top of my background map in the map layer. I then logged out, using the exit game to force a saved copy of what all I had done.. The next time I loaded up roll20 and went to that campaign/page, I found that about 1/3 of the images had vanished from sight. I clicked on the main map/background, selected move to the back, and they all reappeared infront of it. Since then, I have chosen to design any changes to my background maps in photoshop and if a player wants to MOVE said item (box/crate/bookshelf) I can always white-out the background image and place a moveable token in its place. I imagine this saves on number of handles each page/campaign needs to keep track of as well.