This is what the reader understands about Charlie that he doesn't understand about himself. He cannot recognize common images.
This is evidenced in the first line when he tried looking at the image but all he could see or recognize was the ink, at that point, he thought he needed new glasses.
As Charlie got scared when something was written down as he believed he would fail the test, he tried again but could only tell that he was seeing little points of nice ink around the edges. He still failed to identify the image.
