Then you were running Lion or later when you erased the disk, not Leopard or Snow Leopard. So, I would give this a try:
Install or Reinstall OS X from Scratch
Be sure you backup your files to an external drive or second internal drive because the following procedure will remove everything from the hard drive.
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Reinstall OS X: Select Reinstall OS X and click on the Install button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible
because it is three times faster than wireless.
This should install the version of OS X that you had installed.
According to my information your iMac came with Leopard. That means you can install any version of Snow Leopard on a retail DVD. If you have the original Leopard discs that came with the computer, then you can also install it. But you will first need to boot from an installer disc and repartition the hard drive (not erase) because as of now it still has the Recovery HD partition you relied on to erase the startup volume. That Recovery HD should allow you to reinstall the OS X version you erased.
If you wish to revert back to Snow Leopard then:
You can purchase Snow Leopard through the Apple Store:Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.
After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store.
