It is amazing to me what a difference a polished photo or image can make in a blog article. You can see a difference - blogs with great article content but no pictures feel like they are lacking something. It may be that the splash of color or other artistic composition in an image appeals to one part of the brain while the words appeal to another, providing a more holistic experience as a reader.
Companies with a good instinct (or department) for public relations understand this and will provide polished images as part of press kits. For the blog writer who is not a talented photographer, there are also sites like PublicDomainPictures.net and Public-Domain-Photos.com which provide quality photos, free for personal or commercial use. Each also provides photos from sponsors at a cost, but their libraries of free images are sizable.