Mathematica 9 lets you create presentations with background images. There are 6 default stylesheets for this: Earth, Garnet, Sapphire, Opal, Sunrise, Steel and Water.

For your existing presentations I have found that you can apply the new appearance by selecting one of those stylesheets and copying the very first cell in a blank presentation created with the desired stylesheet.
My question is: What is the recommended procedure to change the background image to something else?
I have already found that the background images are controlled by the following cells in the stylesheet:
Cell[
StyleData["Notebook", "SlideShow"],
BackgroundAppearance -> FrontEnd`FileName[{"SlideShow"}, "EarthBackground.png"]]
Cell[
StyleData["FirstSlide"],
PrivateCellOptions ->
{"PagewiseNotebookBaseStyle" ->
{System`BackgroundAppearance ->
FrontEnd`FileName[{"SlideShow"}, "EarthTitleBackground.png"]}}]
And I can indeed put an adiditional background image file inside the $InstallationDirectory/SystemFiles/FrontEnd/SystemResources/SlideShow directory,
but If I do that it will not be portable. So it is possible to use a background image that is contained inside the presentation notebook?