I find it helps to candle some eggs with no development (just grab one or two of each color out of the fridge.) The light will tend to make the whole inside of the egg sort-of bright, and the yolk might be visible (or not, in dark-shelled eggs.) The air cell is at the big end, and brightest of all.
Eggs that have development will usually have darker areas inside. The dark areas change size and shape at various points. When an egg is ready to hatch, there will be the light air cell, but everything else will be pretty much solid dark, because the chick blocks all the light.
The candling photo labeled "#1 a lot like this" has a chick growing in it. So does #20. I think #19 has a chick, but younger than those two. I don't have specific opinions on the other photos you showed.
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