That depends on the Aspect Ratio (AR) of the original image and the AR of the project or final output you require:
image and the project / final output are the same AR then you will have to crop the image if you want it to fill the full width of the screen but have black bars top and bottom.
image AR is eg widescreen (16:9), project / final output is eg 4:3 - then image should automatically be letterboxed.
image AR is eg 4:3, project / final output is eg 16:9 - then image should automatically be pillarboxed - black bars left and right then you have to adjust the width of the image to full screen and crop top and bottom.