CMFCToolBar buttons black when hover

Question

I have a problem with my CMFCToolBar created inside a CDialog. The problem is that when I move the mouse on top of any button, it's background becomes black, and all the colors of the button image are wrong.

This is the code I use to create the toolbar:

CMFCToolBarInfo tbParams;
tbParams.m_uiHotResID   = IDB_PNG_TOOLBAR_AMBIENTE;
tbParams.m_uiColdResID  = IDB_PNG_TOOLBAR_AMBIENTE;

if (!m_AmbienteTB->CreateEx(this, TBSTYLE_TOOLTIPS, WS_CHILD | CBRS_TOP | CBRS_TOOLTIPS | CBRS_SIZE_DYNAMIC, CRect(1,1,1,1), IDR_AMBIENTE_TOOLBAR) || 
    !m_AmbienteTB->LoadToolBarEx(IDR_AMBIENTE_TOOLBAR, tbParams))
{ ... }

And these are the results.

Normal (it's correct):

Hover (wrong, as you can guess):

The toolbar image I load (IDB_PNG_TOOLBAR_AMBIENTE) is a PNG image created with Axialis IconWorkshop.