In order for the image/icon to appear in the statusbar of Firefox, you have to add 1 of the following classes: statusbarpanel-iconic, statusbarpanel-iconic-text and statusbarpanel-menu-iconic. Detail definition of each class can be found at https://developer.mozilla.org/en/XUL/statusbarpanel#Style_classes. Here is a code example that I added in my overlay.xul:
<statusbar id="status-bar"><!-- The statusbar id can't be changed. It should match with Firefox's. --> <statusbarpanel id="id-statusbarpanel" image="chrome://yourExtension/skin/mainicon.png" class="statusbarpanel-iconic" tooltiptext="Label shown when mouse is on." /> </statusbar>