You shouldn't have to flash an iso to a usb in order to run it as an operating system in memory.
When you press f12 and go into bios the bios gives you a drop down menu of the available devices.
In the same way if you select a usb it should ask you if you want to boot directly from the usb OR boot from a .iso file on the usb.
If you select a .iso file it should give you a list of the .iso files present the top directory of the usb. It should also give you a little menu to dive into folders to look for .iso files.