If you just want to flip an already done JPG, Irfanview is probably quickest, since it does so from the initial viewing page. Other programs with a bit more processing power that also flip include Faststone Image Viewer, Photoscape X Pro, and Raw Therapee. Faststone works nicely but does not always read Raw files accurately, and if you try shooting in very low key, for example, it will make a mess trying to compensate.