Didn't went through the entire tutorial line by line. What I suspect though is that you are overlapping your textures by just looking at the screenshot.
Imagine you are drawing pictures on the only piece of paper you have at hand. Suppose there are 1, 2, ..., k images, you draw the first image i1 on the piece of paper. Now, you wanna draw the second one i2. Since you have only one piece of paper, you have to rub away your previous drawing first. Without clearing the drawings on your paper, your new drawings will always overlap the old ones. Unless you are using a new piece of paper everytime, of course.
Back to the question. If the images used in the animation are fully opaque (not even a single pixel), then of course you will not notice the difference even if you draw new paintings over the old ones. But in your case, there are many transparent areas in the images. If the canvas is not cleaned everytime before drawing a new painting, it is obvious that the results will be something similar to what you have in the screenshot.
The images used in the tutorial are fully opaque, I suppose.