Every time I open up a previously made image, usually a jpg of an illustration I did by hand, there is an outline of a rectangle right in the middle of my image. I have no doubt I put it there while doing something else. But, now it shows up in all the images I open in Illustrator. Does anyone know what I might have done to create this, and better yet, how I can get rid of it? I'm using Illustrator CS4.

This thread discusses how to remove the rectangle in the middle of your image.