Common errors

I see only a white square

If you see a white square make sure the height of your image is not greather than the y_position value you set. To bypass that create an image with higher height. Images must also not have Uppercase or Bad Characters (eg. - or _) in the filename.
Keep in mind that the max size of a font image is 256x256. This is a Minecraft limitation. To bypass this (if you're creating a GUI or HUD) you can split your image in multiple font images and merge them shifting them.

When I add a hud others shift of some pixels

Be sure to have each image size set to a multiple of 2. Example: - 2x2 - 4x4 - 6x6 - 52x52 - ......\
If you still have problems try to increase/decrease the size by 2 until the wrong shift disappears. This is an approximation problem I cannot fix.\