Largo's dirt

The following is known:

~Largo calls Dom and Ed saying he needs money. Strip 40

~Ed says, "What? Again!?!" This shows that this has occurred in the past. Strip 40

~Largo confirms to Piro he blackmails Dom and Ed when he brings up some dirt he has on them. 41

~Largo and Piro spend the money on videogames instead of tickets home. 41

~The information must be highly destructive to Dom and Ed for them to pay for plane tickets.

~In the first book, under the comic "Almost Home" Piro wonders what dirt Largo has on Dom and Ed, then he remembers two photographs. Dom then comments by saying, "Lies! You can't prove that's me behind the ewe!" 41

The following is unknown:

~What information or "dirt" does Largo possess?

~How did Largo get this information? (Did he set them up? Did he discover it? Did he witness something?)

~Why has Largo only blackmailed Dom and Ed in Chapter 0?