It turns out there is already a method called Texture for what you are trying to do. You can accomplish the output image with the following code:
using (MagickImage pattern = new MagickImage("connect.png")) { pattern.Alpha(AlphaOption.Set); pattern.QuantumOperator(Channels.Alpha, EvaluateOperator.Set, Quantum.Max / 2); using (MagickImage wallpaper = new MagickImage(new MagickColor("#2980b9"), 1440, 900)) { wallpaper.Texture(pattern); wallpaper.Write("test.png"); } }