Since the advent of Instagram in 2010, Detroit artist and photographer Nick Jaskey has been generously sharing his images with the world, free of charge.Drawn to the subtle imprints of humanity in a large city with a waning population, Jaskey, 35, has long described his photos as a collection of evidence, or "signs of life." But the images that have graced his highly revered 'gram are actually just scraps from the cutting room floor: Stashed away in his house is an amassing stack of boxes, each box filled with developed images that were taken on jaunts through the streets of Detroit over the past two decades.…