я пробовал так, но ничего не вышло
//в IMyIEExtention::CreateOurWindow(HWND hParent)
//присвоил m_hOurWindowParent=hParent
UINT nCount = SendMessage(m_hOurWindowParent, RB_GETBANDCOUNT, 0, 0); //
//у меня nCount=6

REBARBANDINFO info;

info.cbSize = sizeof(info);
info.fMask = RBBIM_STYLE | RBBIM_CHILD | RBBIM_BACKGROUND;
if ((BOOL)SendMessage(m_hOurWindowParent, RB_GETBANDINFO, 4, (LPARAM)&info)) {
info.cbSize = sizeof(info);
info.fMask = RBBIM_BACKGROUND | RBBIM_STYLE;
info.fStyle |= RBBS_FIXEDBMP;

SendMessage(hwndToolBar, RB_SETBARINFO, 0, (LPARAM)&info);
}