The post should have at least one image (with the caption giving attribution to the source). If the blog post is long then space it out with a few additional images. Links to pages or websites the blog post refers to is also helpful.
The blog post should be informative. Either reminding the reader of something they might not have thought about lately or giving the reader some new information they didn't have before.
If the content in the post is primarily a single image then the text accompanying the photo should be as substantial as the image. @minnowbooster's terms of service recommends 150 words which I believe is reasonable. The reader should clearly understand why the image is important after reading the text.
It might also be helpful to include a "call to action" in the blog post and recommend what the reader should do after reading the post. For example, in a recent steemit post I wrote about how I claimed HUNT tokens in the steemhunt airdrop. I referred to a DTube video I had recently resteemed describing how to claim HUNT tokens and I encouraged readers with even the smallest amount of steem power to make a claim before the steemhunt airdrop deadline.
A well written blog post is probably the most essential. It doesn't have to be on the level of a college essay but it also shouldn't look like it was written by an eight year old. The purpose of writing is to get the message clearly from the writer to the reader. Very obvious mistakes are like speed bumps on a road. Too many and the reader will stop reading and walk away frustrated.Photo Source: steemit.com