Installing an image using SDG Image downloader

Hi everyone.
I have downloaded the latest VTI image onto my new Duo+2.
But I want to now change the image to another one. I have used the image downloader to download Open ATV and Blackhole. to my usb stick.
But there does not seem to be a program within the VTI image to install any other image.
Please could someone explain to me how to install and what software or program is needed. I am unable to install an image using my usb sticks as they do not seem to read by
the receiver.
Many thanks.
Stuart.