The best time for harvest can't be determined from pics. Depending on what type of high the plant gives and the smoker's preference, ideal harvest time can be any time within a few-week period. Just from the perspective of our smoking preference, more indica varieties get harvested much earlier in the ripening process than more sativa types; probably at least two weeks earlier, on average. A shorter flowering period gives the indicas a trippier, more fun and less couchlock high. The longer flowering gives the sativas a fuller, more relaxing, less jittery high.
Pop off a tiny piece of bud from your most mature-looking plant and quick-dry it in the microwave. Give it a smoke and decide for yourself how much more development the high needs.