Average is just what is sounds like — take the sum of the observations and divide them by the number of observations. If you want to know what the average age of the people in your shul is, simply add up the ages of everyone and divide by the number of people.
That being said, average does not equal normal. Averages can be greatly skewed. The story is told of the elementary school that tried to attract parents by stating (truthfully) that the graduates of that school averaged a million dollars a year in income. Of course, what wasn’t pointed out was that two of the graduates were the founders of Microsoft, who skewed the average to the right.
To find out if something is “normal,” you have to take into account the average *and* the standard deviation. This gives you a better picture of what is normal.
As for your question regarding how someone could be “average” if we are created B’tselem Elokim, then by definition, about half of us have to be below average. Or, to give you similar example, if you had a list of all the millionaires in the world, by definition about half of them will be below the average of that list. So too with people. If everyone is a Tselem Elokim, then, by definition, about half of them will fall below the average.
The Wolf