One question that commonly pops up is Do you really need a picture for a blog? The answer is no. Imagery is there to enhance your content. Above all the purpose of including an image with your blog should not be to make up for weak content.
Pictures are also more appropriate for certain types of blogs rather than others. For instance a blog about modern art or architecture might require pictures more than a blog about the latest computer chip would.
However there is no reason why the occasional picture cant be used to augment your writing even if you are using a blog to sell a very dry product such as computer software. The true purpose of an illustration or photograph is to enhance your blog, not detract from it. If the image enhances the point you want to make or is even a well-attuned metaphor for what you are trying to say, then by all means use it.
One problem is that some blog templates are not that friendly when it comes to uploading pictures. In this case it is possible to have the image open in a separate web page so it does not interfere with the layout of your blog. This also gives your reader the choice of whether they want to bother looking at...