Should I have to pay for a new screen?

I took my iMac in for a graphics issue. They kept it a few days for testing and I got it back today. When I started it up at home there was something funky with the LCD.

On the boot screen, there are 4 darkened vertical lines, a squiggly line, and sort of a "C" shaped one. I also notice that the screen was not put back on totally straight, as there is a gap where the glass meets the metal on the lower left side. I went back to the store, they check me in, boot the computer and the guy says it's dirt. He takes it to the back to clean it. He comes back and says that actually it's not dirt, the marks are burned into the LCD. He goes on the say that it couldn't have been anything they did, since they never opened it! I told him they had to have opened it because when I got it back the first time the screen wasn't on flush.

Much more discussion and talking with a manager didn't accomplish much. They just maintain that it was never opened because it wasn't documented. They offered to replace it for half price, but that's still $225. Has anyone ever had an issue like this? Am I wrong to feel like I shouldn't have to pay at all?