Adverbs are over-hyped
It turns out that one can always create an adverbial prepositional phrase using the noun:
Aggregate these pictures by the events they may belong to: aggregate by event.
This way you do not need to create a free-standing adverb. You don’t even need an adjective (like event-based, event-driven, eventful) this way, and the simple noun is just fine.
From Wikipedia:
In linguistics, an adverbial phrase is a group of two or more words operating adverbially, meaning that their syntactic function is to modify a verb, an adjective, or an adverb. Adverbial phrases ("AdvP" in syntactic trees) are phrases that do the work of an adverb in a sentence.
When you need to describe how or when or where or why something takes place, all you need is a constituent that functions adverbially. You don’t need a literal adverb.
The only time you truly need a single word of some particular part of speech is occasionally when writing computer programs. In actual language used by actual humans in actual speech, we combine words into phrases to convey infinitely more than ever can be expressed in a single word alone. And that’s not hyperbole, either.