When you choose Fit, it will always scale the picture so that either its width or its height fits the screen, whether horizontally or vertically.
Do you know your picture's dimension? If it is bigger than the screen resolution, you need to resize it before using it for background. If it is smaller than the screen resolution, you want to choose the "center" option instead.
If you need help finding out the picture dimension, locate the picture file in File Explorer, then right click > properties to review the dimesion.
To find out the display resolution, review it in Settings > System > Display.