When an image is uploaded to WordPress, the image is automatically saved in four sizes. This allows WordPress and you to use these image sizes strategically on your website. The WordPress image sizes are setup in the WordPress settings, and applied to every image upon upload.
WordPress images come in four sizes.
The Full size image remains exactly as uploaded, in the original resolution and dimensions.
The largest of the WordPress generated image sizes is Large size. This image is supposed to be big compared to the contents, but loads quicker than an unedited image.
The Medium size is a moderate size between the large and small sizes. The medium size images will keep their image proportions.
The Thumbnail size can be setup in two ways. Cropped and not cropped.
Not cropped will keep the image proportions as they are, and will not crop the image at all. This could result in thumbnail images with different proportions, if the uploaded images are not all the same proportion when uploaded.
Cropped will cut the image sizes to the exact dimensions specified. For example, if the thumbnail image size is set to 300 by 300, and the uploaded image is 600 by 300, 150 pixels will be cut from each side to make a 300 by 300 image.
Thumbnail images are usually setup as a square, but it depends upon your website layout and the intentions of your WordPress theme. Thumbnail images are usually setup using the featured image.
Changing WordPress image sizes
You have control of all four WordPress image sizes. You can setup the image sizes here. http://YOURWEBSITEHERE.com/wp-admin/options-media.php
Or go to “Settings > Media” from the backend sidebar.
Using the smaller image sizes will cut the page speed load time because the smaller file sizes will load faster.
If you change the image sizes in the WordPress media settings, the images already uploaded to your media library will not resize automatically. Existing images will still have the previous sizes applied from when they were generated.
To resize existing images, you need to regenerate the images in your media library. We use the plugin Regenerate Thumbnails.
Resizing images will only effect images applied in the theme files, like featured images. New image sizes will not resize images you applied manually in your content using the text editor.
Learn more about Post Thumbnails in the WordPress codex.