Icon to start the workflow in the item menu

I've created a workflow and chosen the option "Enable workflow to start from the item menu" and define an URL to a picture for "Menu item image URL".

However, the picture is not displayed in the item menu. I see only the label.

Can you please let me know what could be the issue? Is there a specific configuration in SharePoint 2013? Because in SP 2010 it's working fine.

Thanks.