Seeing that all the answers you've received point you to make your own derived button class, I would say that it would be easier for you to see what's already done here in CP, you have a big button resource here:
http://www.codeproject.com/KB/buttons/[
^].
See the Owner draw area.
From there I've used Davide Calabro job:
CButtonST v3.9 (MFC Flat buttons)[
^] which can do what you need.
Learning how he have made it or using his code you'll get your desired result.
Good luck. :thumbsup: