Add Icon

You can do that through some CSS. It would look something like this:

.rt-prefix::before {
  content: url('../path/to/image.png');
}

You can add custom css through your site’s Appearance > Customizers > Additional CSS panel.

You’ll probably want to position it absolutely and play around with the styles a bit.

This would take a little more CSS like:

.rt-prefix {
  position: relative;
  padding-left: 20px;
}

.rt-prefix::before {
  content: url('../path/to/image.png');
  position: absolute;
  left: 0;
  top: -2px;
}

You can play around with the padding-left and top numbers to get it aligned where you like.