Isn't an "image album" basically a prototype used for storyboarding? Maybe it's not a prototype of individual tracks as such, but it is a prototype of style, tone, and the composer's capabilities.
Character songs are a bit more tricky. If they were used in the work, then they wouldn't be image at all. I see what you mean about the broad definition "music created for a work but not used in it" might cover too much stuff, but with this division, the Image Album doesn't really fit under the same definition of Image Song.
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