Blog posts just read better when a photo is included. While a psychologist might argue, my guess is that the effect of a photo is due to our visually oriented minds. We simply like photos, the images give an immediate context for the article that is processed before we even complete reading the first sentence.
The photo can be anywhere in the posting, to either side, across the bottom, as long as it is visible with the top of the article. It helps if the blog layout is clean, without too much visual clutter from other images or advertisements. Even worse is a background image that distracts from the primary article and images.
The extreme example are blogs that include a photo no matter the subject. One of my favorites is Photo Attorney by lawyer Carolyn Wright. This is a great blog covering the legal aspects of copyright law and the business of photography. Carolyn often includes a photo with every post. And while the post covers some legal aspect of photography the image is usually a dramatic wildlife photo taken by Carolyn. The juxtaposition is sometimes interesting, hawks or lions while talking about courts and plantiffs. There is no denying the effect of the photo, you just want to read the posting.
I make it a practice to include a photo in most postings. Even if it means reusing a photo that has been published before. After a while any blog begins to accumulate quite a collection of photographs. This provides a ready set of “stock” photos which can be used to illustrate any posting.