If you are going to be distributing this presentation, there is no way that you can entirely lock somebody out of copying the background. If somebody is determined to get that image, they will. They can make a screenshot, they can extract the contents of the .pptx and find the image file, etc. If you can't risk having the image "escape", then just don't use it, because if somebody is determined, they will get it.
If you were to use flash to make the presentation, you could potentially make it impossible for a user to extract the image, but they could still take a screenshot. The work required to make your own editable presentation software in flash would be ridiculous though.
Basically, your best option is to either use the master slide so that it's harder for users to edit it and to get the consistency, or else don't use that image.