Some image processing and analysis applications require performing image interpolation. This paper focuses on interpolation techniques that treat the shapes and the structures of binary images. A summary of some interpolation methods is presented, and their behavior concerning inclusion relationships and homotopy issues is studied. Then, this work discusses an inclusion relationship property that is used in a technique of ours based on median sets that has been recently proposed. The paper shows that such a property can improve shape interpolation results in a relatively easy manner. Several experimental results are provided.