I'd say the problem you're having is mostly due to the relation between the Axis Center and Maximum Distance setting.
The maximum distance defines at which distance to the center the axis input is 'full', i.e. 1. If your axis center is at X=50 and your maximum distance is 100, full input for moving left is reached at -50, so outside of your actual element (or GUI box).
If your center is at 50/50, you should make the maximum distance 50 or smaller.
Also, if you want the image for the input centered at the axis center (i.e. the center of the image being at the axis center), make the X value at -width/2 and the Y value at -height/2 - i.e. for 100 it'd be -50.