Using custom slide masters on multiple machines

[smp501]

I run the slide show for my youth group, and I have to use my youth pastor's laptop to present with. I make the presentations at home, though, and sometimes the backrounds don't work. We both have OOo 3.1. My problem is that sometimes I will make my own slide masters (the 28 prepacked ones have gotten old), with backgrounds I download elsewhere, and when I save the picture, add it to the master, and test it at home it works like a charm. However, when I present it on his computer, the picture isn't there and the background is white. The picture is on my flash drive, though. Also, if I manually add the backgrounds to each slide at home and run it then I have a huge file, but it works. How do I make it save the picture file ONCE in the master itself and work on other machines? Thanks!