edit: I could see an argument that those two triangles are a consequence of the format of the image and are not quite the same as the other 17 which are part of the fractal structure. For example
if we drew the central triangles on a large piece of paper then there would only be 17 and not 19
You are correct. The next image wouldn't be able to keep the 3x proportions it implies. Following the same pattern, we went from 5 triangles (the top segment) to 17.