I'm a bit unsure of what you're trying to achieve , I think you want to save a copy of the image you used as your icon. If that's the case, copy the image, the smaller one at the top left of the "get info" pane. Highlight it by clicking on it, then press Command+C. Next open preview. Than press Command+N. This will open the image in preview and you can save it.
Custom icons. Please remind me about this in 20 years time when I am enjoying my pension. Maybe I am bored enough by then and like to customise my icons as well.