I prefer this one:
http://openxmldeveloper.org/archive/...mUIeditor.aspx

Granted, it's not as full featured as I would like it to be and it carries
with it a few annoyances. But I do like how it displays and helps manage
custom button images.

There are some other easier graphics-based tools out there, but I haven't
seen one that helps manage the custom button images.