Usually for a popular series, different kinds of merchandise made in an attempt to "cash in" on the series. In the case of ACC, however, some ( and their illusts ) aren't sourced from the essential "root" of ACC itself, and merely made for financial purposes, like any "entertainment products" does ( Truly, ACC itself does NOT belong to any kind of "entertainment products", and essentially NOT belong to any "kind" of things ). Some differences are easy to tell, while others are extremely difficult and almost look authentic.
In galleries below we put merchandise which have same illust/coverart together. The merchandise itself licensed (NAS, Takara... etc) or not (mixed with other characters ,Korean/Chinese ver.. etc) does not bother. We made three albums for this, and the third is for essentially "bootleg" illusts.
LegitimateThis title is actually japanese "正統的な" in english, which means legitimate or orthodox. It's merely to say that: These illusts ( in the two albums below ) are at the same "rank" as the CD, LD/DVD covers are; the particular "rank" which are capable for us to take them as examples to explain the holistic working system of Akazukin Chacha itself. Illusts in the album below has no difference from the screenshots / Cels or Official Setting pictures (only with colors):
And some Ayahana illusts on the Cobalt (novel?), exhibitions, etc..