Can't Dual-Boot Win7 from Bootable USB

I have a USB that's been formatted correctly by Bootcamp Assistant, and a mounted .iso of Windows 7 on it. I've downloaded all of the support programs that Bootcamp offered, and partitioned the hard drive the same way. I'm currently running OSX 10.7.5.

However, whenever I try to install Win7 onto the partition, I get a black screen with the message "No bootable device -- insert bootable disk and press any key".

I'm not sure what's wrong here, and I'd really appreciate any feedback.