They're created from files, as the first two lines of the docs (other than the title) are:

To create a BitmapInline:

perl -MWin32::GUI::BitmapInline -e inline('image.bmp') >>script.pl
[download]

You could create images of "B", "I" and "U" using Image::Magick (google for PerlMagick) which you could then convert, but it's not the most reliable module. I wouldn't count on your users to be able to run it. It's probably easier to create those letter by hand. It'll allow you to fit the letters properly and clean up details that would crowd the button.

Updated: Originally, it seems I only read the first line of your question, so I added to my reply.