CSS for Sitefinity link manager icon is written in Prometheus.css which can be found in the ~/Sitefinity/Admin/Themes/Default
folder. On line 228 is written the style that sets the background position for the button that uses a background image that is set globally for all Editor buttons.
background-position: -1456px -16px;
So my advice is to add the new link manager icon to the Images
subfolder (~/Sitefinity/Admin/Themes/Default/Images) and change the above CSS to something like this:
background: transparent url(Images/YourLinkManagerIcon.gif) no-repeat 0 0 !important;
the Telerik team