It is the artboard. From what I can tell, if you open an image directly in Illustrator it creates the artboard at the same size as the last document you created. If you are working with images in Illustrator, the best option is to create a document at the size you need then place any images in the document.
To fit the artboard to the image all you need to do is select the artboard tool and double click the image. Or select the image and go to
Object -> Artboards -> Fit to Selected Art.