There could be several reasons for the problem, perhaps the most common is that the image is not exactly the right size for the default menu. It could also be how you've placed the image in menu, or there is other code taking precedence. I'd need to see your code to know for sure.
I'd suggest you look in your html and/or css at padding and margins. They could be too big to allow the image to appear correctly. If they are you can adjust them for the image only, and keep the rest as they are. Also be sure the code for the placement of the image within the menu doesn't have any errors.