Check out the Alpha Brisk skin from the 2010 skinning contest (it's open source) to see how he does it

http://www.dotnetnuke.com/Community/DesignChallenge/2010Entries/tabid/1435/Default.aspx

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Chris Hammond
Former DNN Corp Employee, MVP, Core Team Member, Trustee
Christoc.com Software Solutions DotNetNuke Module Development, Upgrades and consulting.
dnnCHAT.com a chat room for DotNetNuke discussions

If you look at the minimal extropy skin you will see that it uses separate stylesheets for the text size, which the user can select.
The used compontent is the stylesheet widget.

<object id="TextSizeWidget" codetype="dotnetnuke/client" codebase="StyleSheetWidget" declare="declare">
                        <param name="baseUrl" value="<%= SkinPath %>css/variations/" />
                        <param name="template" value="&lt;div title='{TEXT}' {ID} {CLASS}&gt;&lt;/div&gt;" />
                        <param name="default" value="MediumText" />
                        <param name="Small Text" value="SmallText" />
                        <param name="Medium Text" value="MediumText" />
                        <param name="Large Text" value="LargeText" />
 </object>

(works in DNN 5)
Here's a very simple demo that uses the same technique to allow the user to switch off the background images (it loads an extra stylesheet for that)
http://dnnskinextensions.codeplex.com...
Should be easy to change to loading different BG images.
(unless you want the user to upload their own...)

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