Okay as much of an idiot as the guy was, he very much Didn't Do That, (owing to the fact he was ALSO dead when that album was made).
Dawn of the Black Hearts (the suicide photo album) was never an official Mayhem album, it was a bootleg produced by a Colombian record label, which eventually became more famous than most of the band's actual albums because the whole Using A Band Member's Suicide Photo As An Album Cover thing, so now everyone thinks it was an official album and that it was Euronymous' idea to use the photo.
The actual sequence of events went like this:
Vocalist Per "Dead" Ohlin commits suicide.
Rest of the band finds him, Euronymous immediately goes to buy a disposable camera and takes some photos of his corpse, but never uses them for anything.
Sometime after Varg Vikernes stabs Euronymous to death, the photos somehow make their way to the hands of Mauricio Montoya, owner of Colombian record label Warmaster Records, who was pen pals with Euronymous and used to trade tapes with him.
(In fact, there is testimony from other band members that it was thanks to tape trading between Montoya and Euronymous that Mayhem adopted significant influences from the style of early colombian extreme metal bands like Parabellum and ReencarnaciĆ³n, which which eventually coalesced into the sound that would come to characterize second wave black metal, but I digress, I just think it's a cool fun fact)
Montoya also happens to have a couple shitty bootleg live recordings of Mayhem songs at the time.
He decides to edit these recordings into a bootleg album using one of the photos as an album cover, and release it through his record label.
Dawn of the Black Hearts goes on to become arguably the most famous and culturally impactful bootleg album of all time, eventually getting famous enough to get an official re-release from the band (without the suicide photo as album cover).
