Why use photos?
Few things are a bigger blog turn-off than the dreaded wall of text. A good image draws attention and makes your site visitors interested in reading more. Photos can also help your search rankings, especially for those doing an image search as opposed to a more traditional search. A good photo encourages potential visitors to click, while your content makes them want to stay and read awhile.
What kind of photos?
Not any old photo will do. For a serious blog, graphs may be most suitable, while a light hearted parenting or craft blog needs photos of the project or similar. A single photo may suffice for a serious piece, while food blogs need several pictures. Photos can also take the place of written content to an extent, which is often seen in craft or recipe posts.
Avoid blurry or badly exposed photos. Free photo editing software makes us all look like pro's behind the lens. If you know very little about taking good pictures, the best tip available is to use natural light when available. Almost any picture looks good in natural light, regardless of the quality of your camera.