Cale's viola was used on several of the album's songs, notably "Venus in Furs" and "Black Angel's Death Song". The viola used guitar and mandolin strings, and when played loudly, Cale would liken its sound to that of an airplane engine. Cale's technique usually involved drones, detuning and distortion. According to Robert Christgau, the "narcotic drone" not only sustains the sadomasochism-themed "Venus in Furs", but it also "identifies and unifies the [album] musically". Of the vocal performances, he believed "Nico's contained chantoozy sexuality" complemented "the dispassionate abandon of Reed's chant singing". In 1966, Richard Goldstein described Nico's vocal as "something like a cello getting up in the morning". The album cover for "The Velvet Underground & Nico" is recognizable for featuring a Warhol print of a banana. Early copies of the album invited the owner to "Peel slowly and see", and peeling back the banana skin revealed a flesh-colored banana underneath. A special machine was needed to manufacture these covers (one of the causes of the album's delayed release), but MGM paid for costs figuring that any ties to Warhol would boost sales of the album. Most reissued vinyl editions of the album do not feature the peel-off sticker; original copies of the album with the peel-sticker feature are now rare collector's items. A Japanese re-issue LP in the early 1980s was the only re-issue version to include the banana sticker for many years. On the 1996 CD reissue, the banana image is on the front cover while the image of the peeled banana is on the inside of the jewel case, beneath the CD itself. The album was re-pressed onto heavyweight vinyl in 2008, featuring a banana sticker. When the album was first issued, the main back cover photo (taken at a performance of Warhol's event Exploding Plastic Inevitable) contained an image of actor Eric Emerson projected upside-down on the wall behind the band. Having recently been arrested for drug possession and desperate for money, Emerson threatened to sue over this unauthorized use of his image, unless he was paid. Rather than complying, MGM recalled copies of the album and halted its distribution until Emerson's image could be airbrushed from the photo on subsequent pressings. Copies that had already been printed were sold with a large black sticker covering the actor's image.