If it's a single image then you could do something like the below.
Code:
.cbProfile {
background-image: url('URL_TO_IMAGE');
background-size: 100%;
}
The above is just an example. If you supplied it an image URL it'd stretch the image to fill the entire profile div. I recommend adding custom CSS to your Joomla template CSS file as it changes less often and less likely to lose your changes on updates.
For further CSS usage please see the below tutorials.
www.w3schools.com/css/default.asp