It has the following CSS classes on it.
cbImgCanvas
cbThumbCanvas
You can use cbThumbCanvas specifically to reposition the canvas if you like. It uses the exact same CSS rules as profiles do though. Specifically the below CSS rules.
Code:
background-position: 50% 50%;
background-size: cover;
background-repeat: no-repeat;
The position can be overridden though if the user uploads an image and repositions the canvas if it doesn't fit within the dimension limits. What you're seeing is just how the background-size: cover; style works. You can find details on how it works below.
developer.mozilla.org/en-US/docs/Web/CSS/background-size