How to add a custom icon or symbol to a flag link? I'm working on a 'sample request' implementation for a commerce site using Flag, Views, sessions and a little custom code.

I'd like to add an icon or a symbol to let people know that an item they are viewing has been flagged.

I'm not sure exactly how to approach this, I can imagine perhaps adding a conditional field that displays an image or check box that is then changed by Rules or ajax, or perhaps something more complicated in JavaScript?

It could really be as simple as adding a checkbox that is checked or unchecked as the flag link is clicked or maybe even changing the css/html on click to be identified as marked. 

Anyway, grateful for any advice here.
Option 1: Show an image beside the Flag text

You can use CSS to add padding to the left of the text and in it show a background image, as shown also in the community documentation about How to show an image beside the text. Here is an excerpt from it (replace "bookmarks" with the machine-name of your flag):

.flag-bookmarks a {
  padding-left: 20px;
}
.flag-bookmarks a.flag-action {
  background: url(images/i-dont-like-it.png) no-repeat left center;
}


.flag-bookmarks a.unflag-action {
  background: url(images/i-like-it.png) no-repeat left center;
}


Refer to the Flag Theming guide for (way) more details, i.e.:


Theming flag links using CSS, which provides a "map" of the HTML that makes a flag link.
Theming flag links using Templates, which explains how to use the flag.tpl.php file in the 'theme' folder that is located inside Flag's own folder, and which is used to generate the HTML for the Flag links. It also explains how you can theme different flags differently, using a flag specific flag--FLAGNAME.tpl.php file (whereas FLAGNAME is the flag's machine-name).
Theming examples, which contains solutions to some common theming tasks.


Option 2: Transform Flag link into a button

You can transform the Flag link into a button, by adding this CSS (quote from that link):

.flag-wrapper.{flag-name} a.flag-action {
  background-image: url('path to the button image');
  display:block;
  text-indent:-9999em;
}
.flag-wrapper.{flag-name} a.unflag-action {
  background-image: url('path to the button image');
  display:block;
  text-indent:-9999em;
}


Notes:


text-indent:-9999em hides the element (by placing it off the screen).
Replace .{flag-name} with your flag's machine-name.