The Essential Diagram Symbol Designer allows you to add small (16x16) and large (32x32) icons to symbol models. When you are not in the Symbol Designer and you select the symbol icon in the Symbol Palette window, the properties of the symbol model are shown in the Properties window.
There you'll see the SmallIcon and LargeIcon properties. You can click on those properties, browse to the file system, and select a graphic to use as the icon for the symbol. When the symbol is displayed in the symbol palette control, the icon used will be the one you selected.
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