Most of the CGs are just defeat scenes for the main characters.
Like mayfly said, there's a shop you access when starting a new game after beating the game once that uses the crowns you find. For three crowns, you can unlock the full gallery; for three more (each), there are also two options where you can play as a more faithful version of the succubus from the Libra Heart games, and a second succubus hunter character (
looks more like a gender-bent NES Castlevania character to me) with no upgrades, for a total of
four playable characters. The CGs you're most likely missing are defeat screens for those other two characters.
For the curious, the other upgrades you can buy (for a number of crowns) are:
- get the Succubus' curse at the start of the game (3);
- unlock all discoverable upgrades for both main characters (3); and
- "immortality" (4), meaning
you cannot fall below 1 HP you won't lose HP at all when you're hit.
The last one makes the "Instant Death" difficulty setting moot, where you normally die in up to two hits.