Blog images

Try resizing a larger version of your image to 560×315 – outside of WordPress. If you don’t have an image editor, you can use this free online one:
https://pixlr.com/editor/

(Or you can use an image editor on your computer. Irfanview is really great little (free) image editor: https://www.irfanview.com/)

Upload the 560×315 version to your WP media area and then add it to your post at full size. Use the dropdown list box for size and choose “full size”.
For more information, take a look at this handy tutorial:
https://en.support.wordpress.com/links/image-links/