They're made with Koala Vinyl Glossy Stickers. They can be applied almost to any smooth surface.
Metal Slug
Metal Slug Fio FOR SALE! - PicClick
Original Sticker Metal slug 2 Neo Geo Snk arcade Video game Part
Aksys Games Reveals Norn9: Last Era Exclusive Card Set ~ Chalgyr's Game Room
Sticker
Tarma Metal Slug | Sticker
Metal Slug Artwork Download - Colaboratory
Snk Stickers
Street Fighter Sticker
Arcade Stick Sticker
Shotgun Metal Slug Pixel Art Sticker Waterproof Metal Slug Retro Game Arcade Pixel Art Sticker Gifts for Retro Gamers Metal Slug Gifts
Metal Slug Decal
Marco Metal Slug Sticker for Sale by KMSbyZet