This is something that frustrated me for a long time. Open Office Impress is powerful and importantly free but it is not the most user-friendly software. These tips were found on nevil jose’s blog. 1) Go to Format / Area (Yes, believe me! We start going to “Format Area” )
2) Choose BITMAPS on the window and click the Import button
3) Name your imported bitmap
4) Go to View / Master / Slide Master
5) Right click on a free area of the slide and go to Slide / Page Setup
6) On the background tab, choose bitmaps from the drop-down menu and select your previously created bitmap.