OS X :: IMac IC2D (plastic, Not Aluminium) Screen With (dark Corners)?

I have a question (whoa). I have an iMac Dual Core, bought in mid-2007 (it's the White Plastic version, not the newer Aluminium with the glass-display).

For some time now I had some dark spots, kind of shadow like in the bottom left corner. I wasn't bothered too much by them, but they got bigger, and now I have these in all of the corners except the bottom right one.

Anyone got any ideas how to get rid of them? (no, no three-year guarantee plan ... stupid me), or if not if it'd mean to replace the whole display or if it's just some cleaning (I talk money-wise here ...)

I attach two not-so-good pictures (the background is plain-white and the image is a bit enhanced ...) so you guys can see what I mean.