Add Icon to Safari Extension

I've written a couple of Safari extensions for my own use but this time I'm writing one for others and I want to add a customized icon for the Extension. The Safari Developer Liobrarydocument says "A generic extension icon is displayed with a label derived from your folder name. Add your extension’s HTML, CSS, JavaScript, and media files to this folder. If you include an Icon.png file, the icon changes to show it, otherwise the icon remains generic.". Well what I did was to convert and icon to a png file and I added it to my extension's folder. I then Reloaded my extension from the Extension Builder but the icon didn't change. What am I doing wrong?