Last month, I changed my desktop background to the Zebras(the picture came with the OS X). In the left-bottom corner, there was some dark color. Today, I saw my screen got some grey area. Firstly I thought it was some dirts. So I picked some paper to wipe. However, I realized it was not dirt. It was something inside. As you can see, the shadow part is the same position as the wallpaper. I just use the wallpaper for less than a month. I contacted apple care today. They said they can come to my home as I bought apple care. But they can only take my iMac away as they can't fix it in my house. I may need to wait a whole week until they fix it. So I ask them not to come as this is my main computer. I choose a light color picture as a new wallpaper. I hope the shadow goes away in one week or ten days. So I don't have to fix it. If not, I will ask the apple care people to come and take my iMac away and fixed. Then I have to use my Macbook air 13 as the main computer for a while.