You can use the Export all Images command under the Document Processing panel to get the image out of Acrobat.
Then you can select a file format like JPEG, that can be used for the desktop background.
If you have Acrobat X Standard or Professional installed you can extract an image from a PDF.Use the extracted image for your desktop background.
Info from Acrobat Help —Acrobat X Standard:http://help.adobe.com/en_US/acrobat/standard/using/WS58a04a822e3e50102bd615109794195ff-7ef2.w.html
Acrobat X Pro: http://help.adobe.com/en_US/acrobat/pro/using/WS58a04a822e3e50102bd615109794195ff-7ef2.w.html