There are a multitude of Reddit threads dedicated to this question. One thread was begun by user el-grosso, who wrote, "Throughout season 3, there is that badass picture of Heisenberg. The twins see it when they crawl into that place, and it's shown again after the twins murder that guy with an axe. With Tuco and his associates dead, and Hector unable to talk.. who drew that picture?"
User architacos remembered Walter's first meeting with Tuco, during which he extracted payment from the drug lord by threatening him with a pound of highly explosive fulminated mercury. "I assumed the image was done by one of Tuco's men," they wrote. "He had plenty of people working for him in that building that Walt went in to get his money from Tuco and almost blew up the place. Someone must've seen him."
Over on another thread, user klownxxx arrived at a similar conclusion. "[Tuco's associates] No Doze and Gonzo both saw Walter in his hat and shades, but they were both dead by the time Tuco kidnapped Walter. Tuco's doorman also saw him [in that outfit], though." User ryderj99 concurred, writing, "Tuco's henchmen saw him. Tuco himself may have sent a description of the meth cook he was bringing across the border."
On yet another thread dedicated to the mysterious sketch, user SquidMan93 posted a response that we believe came the closest to being the correct one. "Tuco saw Walt with the hat and glasses (in the Season 1 finale) and either drew the sketch himself or described Walt's appearance over the phone to the [Cousins] when he arranged for them to pick them up from Hector's house." To that, we say... bingo.
