The cover art type of an image indicates what part(s) of the release it includes.
The front of the packaging of an audio recording (or in the case of a digital release the image associated with it in a digital media store). (link to http://en.wikipedia.org/wiki/Album_cover ?)
The back of the package of an audio recording, this will often contain the track listing, barcode and copyright information.
A small book or group of pages inserted into the compact disc or DVD jewel case or the equivalent packaging for vinyl records and cassettes. Digital releases sometimes include a booklet in a digital file (usually PDF). Booklets often contains liner notes, song lyrics and/or photographs of the artist or band.
(link to http://en.wikipedia.org/wiki/Liner_notes ?)
The sleeve is the outer covering of a vinyl recording or compact disc, it is the paper covering closest in contact to the surface of the disc.
The medium contains the audio recording, for a compact disc release it is the compact disc itself, similarly for a vinyl release it is the vinyl disc itself, etc..
An obi is a strip of paper around the spine (or occasionally one of the other edges of the packaging).
A spine is the edge of the package of an audio recording, it is often the only part visible when recordings are stacked or stored in a shelf. For compact discs the spine is usually part of the back cover scan, and should not be uploaded separately.
Use "inner" for the insides of a release if the packaging folds open (e.g. Digipak or some Double LP releases). Some compact disc releases use a transparent disc tray, use the "inner" type for scans of the image behind the tray.
Digital releases sometimes have cover art associated with each individual track of a release (typically embedded in the .mp3 files), use this type for images associated with individual tracks.
Use "packaging" for pictures of an entire box set, usually these pictures include the outer packaging and the jewel boxes or other contents of the box. For example: http://www.discogs.com/viewimages?release=2943543
Anything which doesn't fit in the types defined above.