The era goes after/below the initial greeting which'll be something like "kinga shinnen" or 謹賀新年 which basically means "Happy New Year".
I made an image one for my online friends last year, so that should help you.

The image can actually be technically anything you want, but more traditional pictures would usually be the animal for th New Year, which in this case would be the Boar/Pig or some other good luck symbol or plum blossoms or something.
There are three flowers that bloom in early spring and those are representative of the New Year because the Japanese used to follow the Lunar calendar like the Chinese did and the Lunar New Year actually starts when spring time is coming. Because of that, the three flowers being featured on nengajou is common. I forgot the exact order, but I think plum blossoms were second and sakura were last. I can't remember what the first one was, but their blooming is symbolic of the New Year so you'll see those featured sometimes.
They also have online nengajou greetings too. Even if you don't send them, just looking at them can help you get an idea of what kinds of greetings you write on them:
http://greeting.rakuten.co.jp/event/nenga/