Your code looks like it should work, except this part when it starts the GS mode.
Thermal.write(29);
Thermal.write(47);
Thermal.write(1);
Thermal.println("Image ...");
"This command is ignored if a downloaded bit image has not been defined."
"GS * n1 n2 d1…dk Define downloaded bit image"
I think you may be using different kinds of modes. There are a few different ones. The GS /n mode requires you to have defined the image, but that is not what you have done yet.
"GS v 0 p wL wH hL hH Print raster bit image" I think this is the one you want to print a raw Bitmap.
I am not positive that I am right. The datasheet is a bit confusing and have several modes. Unfortunately since I cannot experiment with it, you must. 