Or you can find an image you like on the web (be sure it is royalty free, open stock or shared under a creative commons licensing...in short be sure its legal for you to use it) and download it to your computer by right clicking the image with your mouse. A small drop down or window will open with options for what to do with this image.
Most everyone knows how to save images in this way and most will select the "save image as" option and save the image to their computer for use in their blog postings.
But notice when you right click an image on the web to save for use in a blog, there are other options for that image.
One option is "copy image location". If you select this option, it will copy the link location of the image on the web.
You can use that link location to add the image to you blog. Simply click the "add image" icon in the blog editor
Notice the option to add an image from a URL. If you click that, you will see there is a place to add or paste in the link location of the image that you saved when you right clicked the image on the web & chose "save image location".
You will see the image now appearing in your blog.