I have a image of ubuntu that I have used before on a USB flash drive. On my laptop, a MSI GE72 6QD Apache Pro, I am unable to boot from it. When I reboot into the recovery tools, no option to boot from external media is available. I am unsure how to do this. Thank you!
The boot issue from other devices is usually in the wrong BIOS setting. Go to the BIOS configuration. When you turn on your computer and when you see the BIOS-POST tests (white text on the screen), press the DEL key on the keyboard (a different keyboard shortcut on each computer - in your case, the DEL key), and then check the BIOS settings for booting from different devices. Check the boot device order and set up as the first USB device.
Simply search for the "boot" text in your manual for your motherboard or your notebook. The problem may also be the UEFI mode, not all boot devices are formatted to work with the new UEFI mode, so it may be better to activate the "UEFI with CSM" instead of the "UEFI" mode.
English manual (including the BIOS configuration) for your notebook: http://download.msi.com/archive/mnu_exe/1795+1794+16J5+16J4_G_English.zip
You can also use the Quick Boot Menu, but instead of pressing the DEL key to enter the BIOS configuration, press the F11 key. In a fast BOOT-MENU, you select the USB device. But first you may need to switch from "UEFI" to "UEFI with CSM" to make the USB device visible in BOOT-MENU.