PowerPoint uses a concept called "master slides". Your background most likely is in that Master Slide mode.
In PC and Mac version: Go to View-Master-Slide Master
It will then appear. You can change it in the slide master mode and it will effect all slides that have that master slide applied to it.
I find the best file format to export is either PNG or TIFF. test both
In powerpoint you are also able to change the size of the presentation.
By going to File-Page Setup- you can change the size of file. If the graphics of presentation are decent quality, uprezing is usually very easy.
thanks
Michael
www.mdshivers.com