To be a little more clear about the diamond and circle... LASSO is a diamond because it represents the constraint that w_1 + w_2 <= 1. The region of (w_1,w_2)

To be a little more clear about the diamond and circle... LASSO is a diamond because it represents the constraint that w_1 + w_2 <= 1. The region of (w_1,w_2) that satisfy that inequality is a square. Ridge is a circle because it represents the constraint that w_1^2 + w_2^2 <= 1. The region of (w_1,w_2) that satisfy that inequality is a circle.