Using photos as a reference point for your design in Illustrator can make the process easier but that doesn’t mean that all of the tutorials below are easy. Some (like the Scanner Darkly one) will take hours for even those well versed in vector illustration. Most of the tutorials use a combo of Photoshop and Illustrator but the few that only use one program use Illustrator. 

Mosaic Tutorial from Design Spice (uses Illustrator and Photoshop)