I love img2icns too! But, doing it manually is pretty simple. You open the folder.jpg image in Preview. Then Cmd+A to select the entire image. Cmd+C to copy it. Then go back to the folder where you want the icon to show up. Hit Cmd+I to bring up the Info window like you've shown above. Highlight the little icon up in the top left corner and hit Cmd+V. That should do it!
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