Installation is not so simple. This requires you to obtain the PECL install for the id3 library, and according to the installer, there is no pre-built DLL for it which means you need to compile the source for it if you use windows. So the best I can do for help on that is to give you the link for it here: http://www.php.net/manual/en/id3.installation.php
As for *what* it'll have an image in, I'm not sure. PHP's offsets do not directly correspond to the file structure from the looks of it, and the documentation for the id3 makes me question if its even available.
If you have some patience and are fairly good with bit mathematics, you can actually do everything manually by following the specification for the file structures. I found that here for version 2.3: http://id3.org/id3v2.3.0
. There is also a id3v2.4.0 available. Structure appears fairly straight forward. May need some combinations with mp3's directly, but I'm thinking its a wrapper for the mp3 itself.