You will need to be logged into WordPress with at least “Editor” rights in order to do this. Also you need to be using the computer inteface with WordPress because the app version of the media library does not allow photo editing.
- Load the Media Library Page (The Admin (black page) version, not the “lite” (Blue) version) https://wpbells.wordpress.com/wp-admin/upload.php
- Click on the image you want to resize
- Click the “edit image” button
- On the right, reduce the “Dimensions”. Featured Images need to be maximum 300 pixels high in order to avoid being cropped. To avoid sizeways crops, reduce width to 650 pixels.
- Hit the “Scale” button to actually make the change live.
- This will affect all posts where the image is used, without any further changes being necessary, so next time you load the post, the image should be properly sized.
Note – images can also be cropped in the media library editor – select the part of the image you want to display, and then hit the “crop” icon to confirm.