Flash Frame-By-Frame Animation: 8-Frame Basic Walk Cycle
From Adrien-Luc Sanders, former About.com Guide
When you finish those eight frames, your animation should look similar to this. Of course it seems a little odd, stopping mid-stride and jerking back--but that, right there, is a single step. It is not, however, a full walk cycle; it's only half of a walk cycle, a single step. In order for a full cycle, you need two steps--fifteen frames, as your first and last frames will be the same (thus the use of "cycle") and so you won't need a sixteenth. Your fifteenth frame would flow right into your first to begin the cycle anew, seamlessly.