Must you understand the painter to realize that there exists a painting?
Yes. Not perfectly, I admit, but we must understand a number of things. Consider, for just a moment, how we know that something
is a painting. What identifies it as being a painting? Well, the primary method is looking at how the paint is laid down. This isn't just a random collection of colored material, but the paint is laid down in specific patterns that indicate how the painting was performed. More specifically, we can observe the individual brush strokes. And how do we know they are brush strokes? Well, the easy method would be to just watch a painter, but if we assume we can't do that, we might be able to show that the die could be laid down in that particular way by being laid down in a liquid state which then dried. Given the unevenness of the way in which it was laid down, we could then infer that it was laid down by an object that consisted had a bundle of thin, long, flexible strands at its end.
Thus, in looking at the painting closely, we could infer that the painter used paints and a paint brush. We could also infer the speed and patterning of the brush strokes, as well as the thickness of the paint when it was laid down.
So, at the very least, in observing a man-made object, one can infer some or all of the method in which that object was constructed.
Now, having inferred how the painting was constructed, we would want to make predictions as to what we should see in a painting. And here is where things just completely break down: there really is no way to tell what a painter will paint. For a basic example, consider Pablo Picasso. If we looked at this painting:
http://www.partiture.org/in1.html
Would we in any way be able to infer that he would later paint this one?
http://en.wikipedia.org/wiki/Image:L'Accordéoniste.jpg
The existence of a creator thus totally destroys any possibility of predictability. Therefore, the fact that we see no "brush strokes" in the natural world, that is, no evidence of a method by which an entity would have encoded information into our universe, as well as no evidence of fundamental unpredictability in our universe (everything seems to be , in principle, explainable), we, as of yet, have no evidence of a creator.