If you changed the dimensions of default WordPress image and thumbnail, you definitely want the changes to be applied to previously uploaded images. WordPress image size can be set using Media Settings option or from the functions.php file of your theme.
If you go to your WordPress Admin > Settings > Media, then you will be able to assign default WordPress uploaded media sizes. You will be able to set thumbnail, medium and large size for your images. When you upload an image, it is resized and/or cropped. Different sizes of your image are stored. The dimensions match the media settings. But when you change these settings, your uploaded images are not updated or resized.
Media sizes can also be set from your theme’s functions.php file. It is done using WordPress functions such as add_theme_support( ‘post-thumbnails’ ), set_post_thumbnail_size and add_image_size. When you change the settings in these functions, stored media is not updated.
When you want your stored media sizes to update after you change settings from the WordPress admin, you will have to use a plugin. The plugin is titled “Regenerate Thumbnails”.
Using Regenerate Thumbnails plugin to Generate New Sizes of Images in WordPress:
- After activating and installing the plugin, go to Tools > Regen. Thumbnails from your WordPress admin.
- Press the “Regenerate Thumbnails” button and you will be able to see the progress. It takes some time if you have a lot of images and sizes set. Make sure to keep the page open and stay connected when for the re-sizing progress to continue.
- After the process completes, you will have updated sizes of all images stored in your server.