Four different line objects.

 Man made: This is the front of a car. I made it black and white using grayscale.

 Everyday: This is a magazine holder in my house. I also used grayscale on this.

 Architecture: This is the end of my neighbor's front gate. I made it black and white with grayscale.

Sports: This is my bike behind a soccer net.  I used grayscale on this picture.

Best two line picture

 This is a picture of a toyata.  I used grayscale to make it black and white.

This is part of my bike's tire behind a net. I also used grayscale for this.