L'expression du samedi!

Sage comme une image is an expression used to talk about somebody's - usually a child's - calm behavior. We often translate sage by good. It also means quiet.

Une image is a picture. Whatever is on the picture is immobile, quiet. Sage comme une image is literally Good/quiet as a picture.

Quiet as a mouse is an equivalent expression in English.

A mom coming back from an appointment, asks the nanny about her daughter:

Mom: 'Elle a été sage aujourd'hui?'

Nanny: "Oh oui! ...comme une image!"

In another context: you are seeing a group of kids playing quietly together. You can tell your friend:

"Regarde-les! Ils sont sages comme des images!"