There’s a difference between “readable” and “interesting”. I think too often, there’s the idea that “readable” means “simple, boring design and layout”, whereas in reality, the designs that tout readability (I’m looking at you, Medium) are so monotonous that they feel like a carbon-copy of everything else. Readability doesn’t just mean that the text is easy on the eyes, it means that the text grabs and holds your attention.
Don’t use stock images. Not ever. Make sure the image is actually a picture of the thing you’re talking about, not just a reaction image or something in the same general milieu. Make the images personal. Alice Maz uses images wonderfully in her essay on Minecraft economies – it helps that she took all the pictures herself, and that the chosen images augment the text and increase understanding. Just be careful when you add an image — make sure that it’s relevant and facilitates communication. A bad image is worse than neutral: it cheapens your words.
When to Write
Write when you want to record your thoughts and refine them, get feedback, or share them with the world because they seem like they could be useful. Apart from that, I don’t think there needs to be any guidelines around the content of a blog, especially a personal one. Just follow your heart. Your blog will develop concentrations as you publish more.