I've tried on it on showcase and it works fine (just change rf-ico-disc to rf-ico-grid). Are you using any skin? Does the image URL lead to the proper image?
The CSS class rf-ico-grid contains a "background-image" of the grid icon, but the CSS class rf-pm-itm-ico contains a "background-image: none;".
That doesn't matter, the class that is defined later overrides the ones before it. The stylesheet inspector should show you which one of the rules is used.
Thank you for your answer, you made me find the answer.
Somebody in the team overrided rf-pm-itm-ico (in a CSS stylesheet file in an unusual directory) to a new URL and my CSS inspector showed none as the URL led to nothing.
My bad, I didn't pay enough attention to a file that shouldn't be there (for a couple of days). But, thank to you, I found it