Have you ever tried to jailbreak your iPhone? You need to have the IPSW to do this. When you “download-only” an iPhone update, the file is hidden. To find it, use Go>Go to folder in Finder.
Change “macbook” to your hard drive name and “apple” to your home folder name.
Once you get to the iTunes folder, find “iPhone software updates” (or “iPod”). Your newly downloaded IPSW should be there.