Start with this article:
explains VB resource files, which is how you store your toolbar
the following is copied from a recent eng-tips.com post:
Toolbars are made from bitmaps. Each button is a section of the
bitmap (see the API help for bitmap dimensions). i.e. for a small
toolbar, each button is 16 pixels wide, so a 2-button toolbar would be
32 x 15 pixels. A large 2-button toolbar would be 44 wide x 22 high.
You need to design large and small toolbars. They are used when
options are toggled between large and small icons (under customize in
the toolbar menu).
SW does all the dividing and indexing. On the toolbar, the buttons
are separated "by the numbers", i.e. button(0) = pixels columns 1-16,
button(1)=pixel columns 17-32, etc. on a small toolbar.