Why does Impress change the background when I save a presentation?

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Sometimes after I complete a presentation using several different background images on different slides, Impress changes the background of all the slides to one of the images I used. This doesn't always happen, but it happens often enough to be a real annoyance because I am forced to go back through the entire presentation and change the background of each slide to the particular image I want. Of course, even when I do, there is no guarantee that they will not be changed again when I reopen the file later. Libreoffice 4.4.2.2 / English / Impress Ubuntu 15.04 / English