It's been quite awhile since I've used Gnome and done anything in the Gnome Edit Menus, but I think it would go something like this:
Go to Edit Menus. Go to the new menu item you created for eclipse. Right click on the eclipse menu item and select properties in the drop down menu choices. There should be an Icon box in the menu editor box that pops up. Click (or double click ?) on the Icon. You should then be able to browse to whatever icon you want to select.