How to use an image as "percentage"

It’s possible. You’d need to add your own custom CSS to do it, though. Example: https://cloudup.com/cV9LfdbJCwP

I pulled that together using Chrome’s browser tools with the following CSS:

div.wppb-progress>span {
     background: url(https://assets.designspongeonline.com.s3.amazonaws.com/wp-content/uploads/2009/03/p1100884.jpg ) center center repeat-x;
}

You would need to make sure your CSS was loading after the plugin’s CSS or add an !important flag at the end (after the repeat-x).