Make your own. Using your own images is best because you’ already have copyright, and can use such graphics as you see fit. I use my own photos from time to time, but also graphics I work in my Photoshop program.
Use Stock Photos. You can opt to buy stock photos through sites like Dreamtime.com, IStockphoto.com, or Fotolia.com which allow you to purchase graphics as needed often with a subscription based purchase. This can get a bit pricey.
Use Amazon.com. If you join Amazon.com as an affiliate, you get any graphic for products to use in your blog. The trick is finding the right product that doesn’t look like a product to illustrate your point. Maybe you want to use product placement, so that works too.
Use screen grabbing or printscreen methods to make graphics. When I blog about another blog or web site, I use Pixlr Grabber extension for Google Chrome (or Firefox). This allows me to capture a part or the entire web site I want to walk about. I edit it for size and appearance, and insert into my posts. You can find other screen capture type extensions for your browser. You can also use the printscreen button on your keyboard to add to an editor of choice.
If I’ve forgotten any other methods, please post in comments below. These are just a few ways I use graphics in blogs I’ve found effective.