How can I revert to Snow Leopard?
The issue is likely due to the fact that the build of Snow Leopard you're using is incorrect and thus does not have the necessary drivers for your iMac. If it's a Mid 2011 or even Mid 2010 model, the reference release of Snow Leopard (10.6.3) was release before these models and does not have what is needed to use the hardware in your iMac.

If your iMac was purchased before or on Lion launch day, it should have come with some gray disks that will have the correct build of Snow Leopard on them. If you purchased your machine a few weeks after Lion launch, you may not have these disks. 

If you don't' have these disks, I would suggest making an appointment at your local Genius Bar. Both Apple Stores I worked in always had reference releases of all major OS releases within the past 5 years, including a universal Snow Leopard build. They should be able to do a quick NetRestore and downgrade you to Snow Leopard (you will lost all files if they do it this way, so have a good backup!).

Hope this helps...good luck.