This is not possible standard in x5.

Be aware if you set it that mobile phone wil slow down. (I had it and toke it out again)

You need custom code, here is a small tutorial: http://andreweb.nl/x5_test/background-full-scroll.php

Hi we have quite a few templates for website x5 with this feature already included on https://www.x5tuts.com

regards

jamie