Custom cursor
Replace the pointer on your page, give it a click state, and let a pet follow it around.
The pointer and its click
Your page can replace the mouse pointer with an image of your own, and swap it for a second image while the button is held.
128×128 is the largest cursor a browser will draw. Anything bigger is ignored and the arrow comes back, so the box enforces what your page can actually use rather than what it could store.
The cursor pet
The pet is a separate image that follows the pointer around your page. It is drawn as an ordinary picture rather than as a cursor, so it is not bound by the same ceiling and can be larger.
An animated GIF stays animated as a pet.
What a cursor file may be
All three slots take the same files, including a Windows .cur, which is converted like everything else.
A cursor file has no settled type of its own: the same file arrives from one visitor's machine as one thing and from the next as another, so what you picked is read out of the bytes rather than trusted from the file dialog.