XF 1.3 Inserting an Image into thread prefixes

Create your custom thread prefix CSS following this guide: https://xenforo.com/community/resources/creating-custom-thread-prefix-css.350/

Then edit the CSS to add a background image and some left padding to push the text over to the right so it doesn't overlay the image.
Here is the code I have there now:

.prefix.prefixCTA1 {
color: @textCtrlBackground;
background-image: url('https://forums-piratesonlinerewritten.com/data/images/fireskul.png');
background-color: @primaryDarker;
border-color: @primaryDark;
background-size: 25px 25px;

a.prefixLink:hover .prefix.prefixCTA1 {
color: red;
background-color: yellow;
border-color: blue;

But for some reason, the image is not showing at all. I kept everything the same, I just added in the background image url code as well as sizing codes.