I assume you're modding the pictures, not the events?
Off the top of my head, and looking at the Gfx folder:
The Plague and Disease mega events have a movie of a ship flying over water. I've never seen the mega pirates so I'm not sure, but I think they may also have this movie.
Jagged Knife have the image of a ship identified as Juggernaut01 in the Gfx folder.
Dread Lords have one of the invasion movies (the one with ships coming down on a city) identified as "Mini_Invasion".
The "war has expanded" event, called "Allied War" in the english.str file, uses the other invasion movie ("Mini_Fleet") of ships heading for a planet.
Most other mega events use either the "first trade route" movie or the "Meanwhile..." movie. I believe the first is used by the Tourism and Super Tourism events, while the latter is used by the Peacekeepers, sub-space tears (5 speed limit), no range limit, corruption, and organized crime (the one that places spies everywhere) events, as well as by the normal event where a minor race steals someone's techs.
The xenophilia/xenophobia and economic boom/recession events use the Future_city image in the Gfx folder. Most of the Precursor terraforming events like the PQ explosion use the "event_pods" image. The Lucky Ranger and Precursor corvettes use the "ship_found" image, while the Precursor Ranger uses a movie that I forgot the name of (it shows a ship flying over a reddish desert). This is also used by the mega event version of the "race finds Precursor artifact" event, while the normal version also uses the "event_pods".
The assassination event uses an image called "Conquered". The normal pirates use an image called "Colony ship attack" or something similiar. The pop boom event uses "event_wormhole".
Am I forgetting anything? I haven't seen all the mega events yet, I believe, so I'm not sure about those.