The first one can be seen either way. For me, I saw a vase first. In all of the pictures, the black drawing is considered the positive space and the white background behind it is considered to be the negative space. If you look at the vase picture again and imagine the background to be the black portion, you see two faces.
Hope this helps! :)
The best answer here would be pencil.
Artists use pencil to make sketches and create studies. They then use these sketches and studies to create their final larger piece. They use pencil because it's quick, can be corrected, lightweight and easy to use. Before pencils artists would make charcoal drawings but once we had pencils it made sense to use them as they aren't messy like charcoal.
