Can I make a signature image transparent?

To make a signature image transparent use the 32 bit image option in order to support the alpha channel required for transparency. For example:

sigObj.RenderBitmap(filename, 300, 150, "image/png", 0.5f, 0xff0000, 0xffffff, 0,0,
       RBFlags.RenderOutputFilename | RBFlags.RenderColor32BPP |
       RBFlags.RenderBackgroundTransparent | RBFlags.RenderEncodeData);

or in javascript:
flags = 0x1000+0x080000+0x010000+0x400000; //RBFlags.RenderOutputFilename | RBFlags.RenderColor32BPP | RBFlags.RenderBackgroundTransparent| RBFlags.RenderEncodeData);
rc = sigCtl.Signature.RenderBitmap(filename, 300, 150, "image/png", 0.5, 0xff0000, 0xffffff, 0.0, 0.0, flags );