Did you add the spacing to the end of the mpldigits.bmp file?
It should be 176 x 16.

(plus I have no idea on the transparency issue)

[EDIT] I noticed your PNG you posted above is oversized.
You have 10 characters of 16x16, so it should by 160x16.
But it is 168x16. Thats bad.
Your finished font-image NEEDS to be 176x16 including the space.
PS and the "transparent" color of a font is TRUE black. (RGB = 0,0,0)