The Underpass is the most popular spot, so you can usually find epic murals there. If you do not feel comfortable going over one of them, you can always go around the back of the Ice Stadium. The wall there is legal as well. For more information on legal walls in Innsbruck check out: innsbruck.gv.at/page.cfm?vpath=bildung--kultur/kulturprojekte/graffiti&switchLocale=en_US
Is this wall still safe to paint?