The subimages file is a text file, named the same as the bitmap with " subimages" at the end, so if your image is called;
"font.png"
the subimages file would be;
"font subimages.txt"
Internally, there is a line for each character from 32 to 127.
each line is made up of five parameters seperated by a colon;
ascii value of the character
x coodinate
y coordinate
width
height.
Here's a sample of one I made earlier showing the first 8 characters;
32:0:0:64:96
33:64:0:64:96
34:128:0:64:96
35:192:0:64:96
36:256:0:64:96
37:320:0:64:96
38:384:0:64:96
39:448:0:64:96
This is for a fixed width font, each character is 64 wide by 96 high.
You can also stick other stuff in the same image (there's quite a bit of space left) and reference using custom names in the same way
adtest:704:576:320:50
mainlogo:0:576:448:64
toollogo:448:576:256:50
You then access these with the loadSubImage() command.
If you don't want to go to the trouble, just use Hubdule's util (linked to by Baxslash above) , it handles fixed and variable width and creates the subimages file for you.