iMac Pixelated Screen
First, try to boot it off a different drive. It is possible (unlikely) that this is severe data corruption on the drive. You can use another Mac in target disk mode, or you can use internet recovery, or you can use a bootable external drive if you have one. 

If you boot another drive and still have the graphic distortions then it is certainly a logic board issue. If it functions normally (this is not likely) then you just need a new hard drive, or maybe just a wipe and reinstall. It's unlikely but worth a shot.