You can change images within the content by clicking on the image and then selecting Replace. You can Open Media Library or Upload your own image.
Add image block
If you want to add an image where there isn’t one currently, click the [+] sign to add a new block, type in “Image”, and select Image. You can Upload or drag/drop, choose from Media Library, or Insert from URL.
You can assign a Featured Image to any Blog post. This image will appear on your Blog archive page as part of a list of all of your Posts. To assign a featured image, go to Content > Posts, and edit any Post. In the right-hand menu, under the Post tab, you will find a section devoted to the Featured Image. Click the “Set Featured Image” button and choose an image from you Media Library. If you do not see the right-hand menu, click the Settings icon in the upper right of the Editor screen (gear icon).