Hey Tim,
You don't do this in the "Customize Toolbar" option, you literally just hold command and then click and drag the application icon from your Applications folder into the Finder toolbar. Works with any application and is extremely useful for quickly opening the files you frequently work with.
The reason this is the preferred method is that it shaves off time required to quickly open images in preview. Yes there are other ways, but they are just slower, require more clicking, dialogues, etc. If you take the shortest route to work every day and someone puts up a road block and then says, "can't you just take another street?," how would that make you feel?
HItting spacebar to preview an item is not an alternative to have the image in Preview, for numerous reasons. If you need the image open for a reference while you build another comp, or if you need to quickly see a high resolution image at 100% scale, or if need to quickly toggle/compare multiple images (you used to be able to select them all and then drag them over the icon).
I appreciate your responses, but I don't need to justify why I use a feature of OS X that is no longer working, I merely want to point out that it isn't working any more, and hopefully someone can enlighten me as to how to get it back.
Thanks 🙂