Your blog has its own image library. Open it from "Your images" on your blog, or upload straight from the editor's picture button while you write. Each image needs alt text (a short description for readers who can't see it), and you can add a credit if someone other than you produced the original image.
Your library holds up to 200 images, each up to 1 MB. When it's full, delete images you no longer need to make room. Deleting an image also removes it from any posts that use it.