In WordPress, images can be displayed in certain sizes based on preset definitions. The term “thumbnail” refers to a smaller version of the original image based on an old term that means “small” because it referred to painted pictures that were approximately the same dimensions as a human thumbnail.
Whenever you upload a photo to WordPress, the system will automatically optimize the image according to several preset size categories. Collectively, these are known as thumbnail size because they are all smaller than the original image (WordPress does not allow you to display an image in a larger size than the original image). This is not to be confused with the “thumbnail” label, which is the very smallest preset size category.
To change your thumbnail sizes, log into your website as an administrator and click on “Settings” in the left-hand column. From there, click on “Media.” From there, you can choose all of the additional (and smaller) image sizes, including the very smallest, which is usually known as “Thumbnail.”
Note: If you change the additional preset sizes (or thumbnail sizes) for your images, this will not change the size settings for any previously uploaded images. If you want to retroactively apply the new thumbnail size settings for all of your uploaded images, you can use a plugin like “Regenerate Thumbnails.”
Keep in mind that WordPress is making copies of each original image for each different smaller image size, and this can use up a lot of storage space. If you find that you’re using a lot of space for storing images, consider reducing the number of different thumbnail sizes generated for each original image. Alternatively, you can pay more to your web host in order to get more storage.