2008 MacBook Air Can't Boot OS X Snow Leopard USB Install Drive

I have an original 2008 MacBook Air (MB003LL/A / MacBookAir1,1 / A1237 / 2142), and I am trying to reformat it with OS X 10.6 Snow Leopard, using the retail install DVD image snowleopard_10a432_userdvd.dmg restored to a USB flash drive with Disk Utility. I do not have an external DVD drive to use.

Whenever I try to boot to the install USB, I get a "no" sign after a few minutes of waiting after the Apple logo. If I boot the install USB in verbose mode, I get:

USBMSC Identifier (non-unique): AF429H02YE09000451 0x154b 0x7a 0x1100

AppleIntelCPUPowerManagement: initialization complete

Still waiting for root device

Still waiting for root device

Still waiting for root device

...and so on.

Does anyone know how I can solve this issue?