1 Image Per 200 Words
Some people fill their blog posts with photographs and graphic images. Others use none. The general consensus is to use 1 image per 200 words. To be honest, I don't always follow the general consensus. For example, when I post a facts only tutorial that does not require illustrations or screenshots to explain the subject, I keep the post bare of images. There's no fluff and strictly fact and how to. As a researcher, I personally like these posts because they are quicker to pull up on screen and I can easily forward the page to my email account to read over later without having to draw in all the extra images.
I also like to change things up a bit. Instead of posting two images for a 400 word article, I might post one image and then link to a YouTube video to count as my second image.
Optimizing Your Images
When you add images to your blog, you have the option to fill out the name, description, caption, and alternative text for each image. This is another way to optimize your blog, so use it. I know from working as a writer on About.com that all the images I keyword optimized show up in Google image searches. This, in turn, brings more people to my little corner on About.com.
The Curse of Too Many Images
I remember the days when people were creating monstrous websites on Tripod and Geocities. These websites were so loaded with photographs, animated gifs, and twinkling backgrounds that it felt like it would take all day to load over a single page. These was also the days of dial-up internet, so pages would literally take a good 10 to 15 minutes to load up if they were littered with crap. While dial-up is mostly dead these days, it is still best to think of your readers first. Some of them may be stuck on a crappy satellite internet provider that chokes every time there are images to load over.
Keep your images to no more than 1 per 200 words and resize your images so that they are easier to load over.