I want to buy the 27", and run a program that is going to be displaying images for many hours a day (Brian Eno's 77 million paintings). I have a concern that running this for almost 24/7 may significantly shorten the life span of the screen... is that a valid concern? I mean, if the screen luminosity goes down 10% in 3 years, I'm not worried, but if it's something like 50%, that would be bad. I'm hoping it would last me some 5 years or so. Is this an issue? How durable are the screens on the 27" iMacs?