Final answer:
Pin pictures are distinctive because they always have straight lines, exemplifying stability and visual direction as seen in art, similar to the straight lines found in architecture within paintings like Las Meninas.
Step-by-step explanation:
Ezra's teacher is discussing an element of form in art, focusing on the nature of lines used in pin pictures. Based on the information provided about the character of lines in various art examples, pin pictures are characterized by having straight lines. From the description of lines within different art contexts, we understand that straight lines provide structure and stability, implying that in the context of pin pictures, the lines involved are straight to offer that same stability and visual direction. For instance, in Las Meninas, you can observe straight lines in the architecture and various supports throughout the painting. Therefore, the images on the handout with pin pictures that Ezra's class received would have this element of straightness as a definitive feature.