Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".
I think you'd be better off not using the Draw methods. I don't know how to code this in C++ but this C code composites a transparent signature over a non-transparent base image. Maybe you can translate it to its equivalent in C++.
// Read the base image
m_wand = NewMagickWand();
MagickReadImage(m_wand,"logo:");
/* Read the signature image */
a_wand = NewMagickWand();
MagickReadImage(a_wand,"sig.png");
// Composite the signature over the base image
MagickCompositeImage(m_wand,a_wand,OverCompositeOp,0,0);
/* Write it out */
MagickWriteImage(m_wand,"logo_sig.png");