Looks like you have dead bugs on the screen (!)
There's air space between the glass (which is the outside surface of your screen) and the actual LCD screen - room enough for bugs to crawl in and die. There are various places where they could crawl in.
You can remove the front glass panel, and carefully clean the enclosed surfaces, which should get rid of those dead critters!
Some instructions here:
http://www.ifixit.com/Guide/Installing+iMac+Intel+27-Inch+EMC+2309+and+2374+Glass+Panel/1631
I use isopropyl alcohol to clean the inside glass surface, and a glass cleaner, such as Windex (with vinegar - not Ammonia (check the label)) to clean the real surface of the LCD panel.
Check carefully for any smears/streaks before you place the outside glass back into place. I always caution folks who do this display glass to _not_ move around too much, so that dust does not stir onto the inside surfaces.
And (as WilliamG already said) you should be bug-free!
BTW - the suction cups don't have to be a lot of money, just something that will hang on to clean glass so you can lift the glass up and out. You can often find something that works at a kitchen accessory shop. I found suction cups that are sold to hold towels or pot holders that work for your purpose just fine.
One final point - when the glass is removed, you'll see that it's very thin, and untempered, which makes it quite fragile - so handle carefully - don't get in a hurry when removing or replacing the glass. good luck!
Let us know if that fixes it for you…
And, one more - don't cover any holes to prevent future "entrance" as you could block needed cooling for your iMac!