How to release brakes to install front tire
The intended method is to unhook the cable from the far cantilever in the photograph. Hopefully the straddle cable end fitting has a wedge-shaped bit sticking out of the back of the calliper, as visible on the far left here: .
Squeeze the two brake arms together, and pull on this wedge, and lift the metal cylindrical part out of the slot. If there is no wedge shaped piece, push the cable into the caliper to free it. Some straddle cables have a piece of metal on the cable to help you do this but I can't see one in your photo.
There is video and pictorial explanation here: http://www.madegood.org/bikes/repair/open-a-cantilever-brake/