I have heard the term used to refer to two different phenomenon, both of which apply to anime soundtrack albums:
The more common usage, refers to an album where the music contained on the album never occurs in the series, movie, or OVA whose name is attached. Often this is a ploy to sell more records by slapping the name of a popular franchise on music that has nothing to do with it. The justification for using the name is usually one of two things:
- The music was created by the same people (as the OST)
- The music was "inspired" by the anime (read: OST remixes)
One classic example is the
Neon Genesis Evangelion image album
Vox. Of the ten or so albums marketed under the
Eva name,
Vox is the most universally detested. These type of image albums tend to be not as high quality as the OST, though there are exceptions to every rule.
The other type of image album refers to an album made up of image songs. An image song is a theme song of a character, which is sung by the character's seiyuu (voice actor). As a result, these songs are often not in the anime, because most seiyuu can't sing very well (Megumi Hayashibara, Maaya Sakamoto, etc. are exceptions, not the rule). An album full of image songs results, hence, an image album. Fushigi Yuugi has a number of these, some quite dreadful.