This problem might occur because you've used an image file that was saved with the CMYK color mode. CMYK is typically use for image that will be printed. Images that will be displayed on a computer use the RGB color mode. Here's an article that explains the difference in a bit more detail.
Solution: To fix the problem, convert the image to RGB. Most image editors will allow you to change the model and save the image.
- To tell which color mode is used, on a mac, open the image in preview and in the inspector look in the General Tab and it will list the Color Model.