Hello wonlon!

According to a classic Орфографический словарь (Spelling dictionary) the correct spelling is поутру. And the stress is on the last syllable.

As far as I can see it's not an official NG article, but some blogger style comment on pictures. His/her phrasing is stylistically not impeccable. Well по утру is just incorrect spelling. So such authors and such comments should be always taken with a grain of salt (when it comes to correct spelling, style, grammar, etc.)

The neutral and most commonly used variant is утром (in the morning). You might encounter поутру in poetic texts, novels, so in terms of register this word is a bit more bookish.

Later I'll provide the link to a Spelling dictionary that can be downloaded (I would like to check some).