The easiest way to do a background selector is through different ResGroups for the one image.
That is the way I did it in Ev0, its a lot easier to manage and if you ever want to add more backgrounds in you just add a new resgroup rather than add a whole bunch of new resources and <PlusSkin> conditions.
<ResGroup>
<Restrictions>
<MsgVersions>
<Version Major="8" Minor="5"/>
</MsgVersions>
<SkinOptions>
<OptionSet>
<Conditions>Background= "Blah"</Conditions>
</OptionSet>
</SkinOptions>
</Restrictions>
.........
.........
</ResGroup>
Anyway if you need clarification just ask.
/ 
