UNetbootin allows you to create bootable Live USB drives for Ubuntu, Fedora, and other Linux distributions without burning a CD.
Unfortunately it does not work by default with Alternate Ubuntu Images. In order to make it work:
1. download an alternate image: Ubuntu Alternative Downloads
2. write an image to USB drive using UNetbootin
3. boot from this new USB on the target host
4. select the menu item “Default”
5. DON’T press ENTER
6. press TAB
7. add the “cdrom-detect/try-usb=true” before the “- -”
8. press ENTER
Now it will actually look into USB to install the system rather than looking for CD-ROM or trying the whole OS from the network.