There is a significant difference between an image and a turtle wearing a shape/costume

An image is 'paint'. A turtle is a programmable object.

An image is like paint on a canvas. It goes on just one layer. It is not an object that can be moved. It can be erased with the cleargraphics (cg) command.

An image is made in two ways: 

1. by using the stamp command to stamp the shape of the turtle 

2. by giving directional commands (fd bk etc.) to a turtle that has its pendown (pd) 

We shouldn't call a turtle an 'image' if it is wearing a shape/costume. 

A turtle can be made to wear a shape from the Clipart Panel by using the setshape (setsh) command. 

It looks like an image—but, it's not! It's an object! 

Yes, it can be used to stamp an image. But that turtle is an object and behaves differently.