Yes, and? Presumably the uncited cut-and-paste from a (poor, IMO) dictionary is intended to suggest that the third definition applies, but nudity isn't sexual activity [0] and, more to the point, the sense of “graphic” in the that definition is basically a combination of vivid and explicit in the sense of the first definition, and remains an antonym of “implied”.

[0] which is more of a problem with your choice of authority, because most dictionaries wouldn't limit the relevant definition this way.

Copied from Google (who pull from Oxford), but a variation of "depicting nudity or sexual activity" is included in most if not all definitions. Do you have a source you prefer that contradicts this?

In common usage, an "explicit image" implies the third definition. It's not explicit as opposed to blurry or abstract, it's explicit as opposed to non-sexual. Since the context was discussion of the contents of an image, the word "image" was implied.