every skin doesnt use a screensaver, of those that do, not all use a particular format. some are wmv (videos) others are skin elements. You would have to open/edit each skin you use to set them the same, or to see what type of saver is used and how it is done.
for instance, you can normally find the saver element near the bottom of the skin.xml.. and it would look something like this for video.
<Layout Screen="Saver">
<LayoutElement Type="Image" Source="Logo">
<Pos l="0" t="0" w="640" h="480" />
<Scale x="true" y="true" />
<Justify x="center" y="center" />
<Proportional>true</Proportional>
</LayoutElement>
Near the top of the skin.xml..where 99.9% of the resources are defined you would see the resource "Logo" is defined as
<Resource name="logo" type="image">
<File>mediaSaver01.wmv</File>
</Resource>
Get the picture?