I would be very reluctant to use the word avatar in this context. I would much rather call it something like a blog post icon or graphical handle instead. Here is why:
Some decades ago some computer programmers were looking for a short word for a very small image. They settled on the word that describes a holy picture of a saint in the orthodox church: icon.
Then more recently, someone was looking for a word for a specific type of icon (in the technical sense) that specifically represents a person. The problem was that the original meaning of icon was lost so completely in a computer context, that even the connotation of depicting a person was gone. They solved this problem by generalising another, similar religious term, that was already used in 3D computer games. An avatar in the original sense isn't actually a picture at all, but it has the other important aspect that icon had lost: it represents someone (a god, not a human) in a place where they can't exist in their usual form (the normal world, not heaven). Now in the figurative sense, an avatar represents a human (not a virtual character) in the virtual world (not the real one). When the word was first used in computing in Ultima IV, the original meaning applied according to the story line, but in reality it was the second. Just like avatars in Ultima IV, the next uses of the word did not apply to images. But as they were in the context of 3D games, they applied to 3D character models. Calling the images associated with posters in a forum avatars was the result of a natural generalisation.
Now it would be totally normal for people to run away with one aspect of a special case of the new usage (a little picture representing someone or something uniquely) even though it has absolutely nothing to do with the metaphor that made the word originally applicable in a computing context. This is just how language evolves.
But many who understand the connections would be reluctant to do it because losing track of etymology is one of the factors that make language so complicated and different languages so different. Just think of the poor Indians who know precisely what an avatar 'really' is: a god walking around on earth. If they learn that another word that is spelled the same way also refers to an image representing a blog post, they may well believe it's a totally different word with an unrelated etymology that just happens to be spelled the same way.
Well, most people wouldn't say it with these words. The more common, and more primitive, way of caring for language in this way is to subscribe to the etymological fallacy. Not necessarily by really believing that etymologically related words must have similar meaning. But by willing this to be the case - by arguing as if it were true, in a doomed attempt to stop language change.
More practically, the best words are those that have all the right connotations and no wrong ones. But lots of the connotations of blog avatar are red herrings, and only a few, though admittedly strong ones, are to the point.