To over simplify, explicit means genitals and/or sex. Questionable means exposed boobs. Suggestive means something children probably shouldn’t see (like overly sexy clothing, or lewd text). A lot of posters use questionable for things which could be called suggestive, just to be safe.
In this case it’s a very nice image of a fully-clothed Twilight, tastefully done. I don’t think anyone would object to their kids seeing it.