As the footer and background image divs are set in the projection screen as absolute position with 0 px top, on screen change the background image and footer is jumping, as it first gets loaded and displayed, and repositioned/rescaled afterwards, thus causing a visual jump. Attached a screencast, which shows the exact problem (please note that it does happen always, but the screencast doesn't show the bug for all screen changes, only for some, because of the 15fps framerate).
Another note: this affects Linux only with desktop environments having a top bar on the screen we are projecting to (e.g. unity on Ubuntu, doesn't happen with Gnome Shell)
As a workaround I have found that in openlp/core/lib/htmlbuilder.py removing the position, left and top css attributes from .size solves the problem, however causing the image to be cut off at the bottom, but for me at least this is an acceptable price to avoid the more visible visual jump. (there's still a small issue with the footer jumping, but I will investigate that, or at least can workaround by setting the footer font size to 0)
So does this only happen with Unity?