How do I add a single picture to a page or post?
1) Hover your cursor over the (+) symbol and select the WordPress image icon as shown below, or you can select the (+) icon and select the WordPress image block as shown below:
2) Within the image block, select 'Media Library' to open the media window.
3) Select 'Upload' and drag and drop your image(s) to insert into your post or page.
4) Select your image(s) and 'select' button to insert into the post or page.
5) After your image is added to your post or page, you may now align the image using the alignment icons above.
For more information on adding and arranging images in your post or page, follow this helpful document from GoGutenberg.