The picture should be about the same size as the start menu / taskbar. Pictures can be both resized and cropped to fit using the built in Microsoft photo editor. When you open up the picture just kind of compare it's size to whatever you have adjusted as your start menu size. If a picture is 1/2 the size of your taskbar that is an easy fix, either crop or resize the photo's height by 1/2.
To Crop go to edit image

To Resize right click empty area and select resize

Then open photo and compare its size to the menu and see if it matches up.