To create square and rectangle thumbnails, WordPress crops images when you upload them to your website.
You should use high resolution images. Default square thumbnail size is 600×600 px. Default rectangle thumbnail size is 600×400 px. So for example if you upload a 300×400 px image, square and rectangle thumbnail options will not work…
If you have a lot of images in the WordPress Media Library and if you don’t want to upload them again you can use the following plugin to regenerate all your image thumbnails in a few minutes.
Upload the following plugin to your website;
Using the Regenerate Thumbnails Plugin is so simple. Just activate it and click “Regenerate All Thumbnails” button. Plugin will regenerate all your thumbnails (They must be bigger then 600×600 px).