Guests can not see images in the messages. Please register in the forum.
Firewood is usually chopped up hardwood that was not of good enough quality to cut into good lumber. But usually, the wood isn't all bad, and individual pieces between the knots and defects can be picked out of a pile that are of good quality, and square.

Check out how at: http://woodgears.ca/reclaim_lumber/index.html
Also
Splitting logs for woodworking lumber