The background color is important: ScanIt is able to tell the difference between symbols or curves and the background because it has a notion of the background color.
By default, the automatic background color detection is on. This is the advised setting. This means that if you load, crop, or filter your image, the background color will automatically be redetected.
If the background color is not set correctly, snap, trace curve and scan for symbols may not operate as they should. To fix this, manually set the background color for the image by using the Background color menu of the Image menu.
This will pop up a window that allows you to: