We've created a simple landing page - www.coworkcambridge.ca using the website builder but we're not able to see the theme image on mobile devices (i.e. smartphones).
I use / used this place for such things http://www.mobilephoneemulator.com/ I just bought a mobile device just for checking the websites..... that place will allow you to see what folks see on their device so it is handy!!
Wish you well,