If you see the following page

http://www.android-x86.org/documents...howto#Advanced

you'll see it mentions an android-x86-1.6-r2_usb.img.gz

Now, if you click on the download link, you'll see there are several .img files there. In the commentary beside its name, it says EFI image. Are these the USB images mentioned above?

Yes, they're all USB images. But not all of them will boot on every computer. If you have a traditional BIOS (or a UEFI chip which has been set to work in BIOS mode), then the boot medium must have some bootloader code in the first sector (the master boot record or MBR). That's how all PCs booted until recently.

If you have a UEFI running in its native mode, the bootloader has to be in a special partition called the EFI System Partition. There's still a MBR but it's just a dummy and the UEFI chip won't even look at it.

The android people don't know what kind of computer you have so they provide different images.

That is to say, the EFI images are for BIOSes running in the UEFI mode?

It is possible to create images that run on both BIOS and UEFI mode. For instance, SystemRescueCd boots in UEFI mode if UEFI is present and in BIOS mode if it's not. It also will run in 64-bit mode when hardware supports it and in 32-bit mode when hardware is incapable of 64-bit.
So you should check out what works for you.

Thanks a lot.

Not quite. A BIOS can't run in "UEFI mode". A BIOS is a BIOS and it can't behave like anything else. A UEFI is a different kind of chip altogether, a rather more intelligent one. The latest computers have a UEFI and not a BIOS. But for the sake of compatibility, UEFIs have the ability to emulate a traditional BIOS. That is the case now. It may not always be the case in the future. As you probably know, "legacy" interfaces tend to drop out eventually.