When you switch to a new theme which uses post thumbnails, there is occasionally a situation that all the old thumbnails look ugly and distorted. Even if you try to adjust image sizes in Settings → Media, it doesn’t solve the problem. That’s when you need to regenerate thumbnails or new image sizes in WordPress.
By default when you upload an image to WordPress, it creates different sizes and saves them in the uploads folder. Many WordPress themes take advantage of this feature and define new image sizes, but the problem is that these new image sizes are only applied on the images you upload after activating the theme. To solve this problem you need to generate new sizes for all previously uploaded images. This process is often referred to as regenerating thumbnails.
To generate images in newly defined image sizes, install and activate the Regenerate Thumbnails plugin. You just need to search this plugin in the Plugins → Add New section and click the Install and Activate buttons to put it into effect. If everything goes successfully, you can find it in the Installed Plugin as follows.
After activating the plugin, go to Tools → Regen. Thumbnails. As is showed in the following screenshot, you can regenerate all images in the specified sizes by clicking the Regenerate All Thumbnails button. Note that this thumbnail generation is irreversible, but you are able to covert thumbnail dimensions into the old values by clicking this button again.
Pressing the regenerate thumbnail button will start generating new image sizes defined by your theme or in Settings → Media page. It takes a while to generate all thumbnails especially there are too many old uploads. You should be patient during the whole process. At the same time, you also can pause it by clicking Abort Resizing Images option at any time.
Alternatively, you can also regenerate image sizes for individual or multiple images in Media → Library.
Regenerating images sizes does not affect your original upload. It creates additional copies of images in the newly defined sizes. It also does not delete previously created image sizes. You can delete the sizes if you are sure that you haven’t used those sizes anywhere on your website.