For us with an OEM Windows 8 and have upgraded to 8.1 via the Store there is no way of obtaining an image of 8.1. I have upgraded Win 8 to 8.1 twice and both installations eventually became unreliable with OS File errors.  You need a key to download an image and the OEM one does not work so Microsoft are out and according to my communication with Dell they do not yet have an image for those on the upgrade route. Install Win 8, then Win 8.1 upgrade them find hassles with drivers and reinstallation of all Software. I fell totally let down by Microsoft for the way we have to handle 8,1 reinstallation and by Dell for not making a downloadable image for recovery available for us.  If you run into any trouble with 8.1 and it asks for recovery material you are up s--t creek without a paddle, there is nothing you can do! Also getting the AMP Catalyst to work in Win 8.1 is also a real problem and so is Windows Fax via a voice fax modem.
To clean install Windows 8.1 .iso using a Windows 8.0 retail product key follow the instructions here: Download Windows 8.1..iso And: Windows Reinstallation Guide/A Clean Install of Windows 8.1 Unfortunately there is no current solution to download a Windows 8.1 .iso for a preinstalled (OEM) version of Windows 8.0 or 8.1 You may also want to have a look at customising Windows 8.1: [URL]
Download Windows 8.1 .iso