In this lesson you will learn how to regenerate the thumbnails for your image attachments when you need to resize images to fit a certain theme.
- Basic knowledge of installing and activating WordPress plugins
- Managing Media
- Creating WordPress posts
- Regenerate Thumbnails plugin
- Twenty Sixteen theme
- Pictures to resize: these two (monkeys, boats) were used in the demonstration, you may choose any other pictures as well.
- Are you familiar with the concept of a plugin in WordPress?
- Have you been working with media in WordPress?
- Do you have a self-hosted WordPress website?
- Do you have a user role that allows you to install plugins?
- The recommended way to approach the scenarios would be to demonstrate and explain the process first and then ask students to repeat the actions using their own devices, while you’re available for questions and troubleshooting if something doesn’t work out.
- It is easiest for students to work on a locally installed copy of WordPress. Set some time aside before class to assist students with installing WordPress locally if they need it or, if possible, send them out instructions before the class so they would come prepared. For more information on how to install WordPress locally, please visit our Teacher Resources page.
- The preferred answer to the screening questions is “yes.” Participants who reply “no” to questions #1-2 might require a bit of explanation, and if they answer “no” to questions #3-4 they may be grouped with other students to work in pairs on the installation.
- You may print out the Hands-On Walkthrough part to use it as handouts or send it out as a .pdf file to keep it green and preserve the links used throughout the document.
A problem that WordPress user can bump into one day is that their featured images appear cropped and out of proportion. This happens a lot after you have changed your Wordpress theme: normally when you upload images to WordPress they are being resized to the theme's requirements. So after you've changed it you may find the thumbnails for the previously uploaded images distorted as they have been sized according to the old theme's requirements. To remedy the situation, one can use Regenerate Thumbnails plugin. Essentially it enforces creating copies of the old images sized according to the new WordPress theme's settings.
By default, WordPress gives you three image sizes that can be customized. They can be accessed and modified via the WordPress admin dashboard under Settings > Media section. These sizes are all generated when a new image is uploaded through the Media Library, so if you check the uploads folder you will see something like:
- picture.jpg
- picture-150×150.jpg
- picture-300×300.jpg
- picture-1024×1024.jpg
So here you see that in addition to the original picture.jpg three sizes were generated (and some extra sizes can also be registered via some of the plugins and themes). Those will be used further on in galleries, posts and for other purposes on your site. 
Install the Regenerate Thumbnails plugin by Viper007Bond and activate it. 
1. Create a new post and choose to Add Media to it. Upload a picture to the media gallery and put it in the post draft twice. Make sure first time it's posted as Thumbnail and the second time as Medium. 
1. Now edit the media settings of your images. Go to Settings > Media section and change the Thumbnail settings to 200x200 and Medium size to 400x200. Choose to save the changes. 
1. To finally enforce regenerating the thumbnails for your site’s needs pick Tools > Regen. Thumbnails from the navigation menu on the left. 2. Click Regenerate All Thumbnails to regenerate new image sizes as defined by your theme or on the Settings > Media page. 
When you need to regenerate sizes only the certain images, you can do that in Media > Library section. To do this you would need to click Regenerate Thumbnails link near the image you want to alter - and make sure you've switched to the list view first.
Other possible choices to regenerate thumbnails would be AJAX Thumbnail Rebuild or Force Regenerate Thumbnails plugins. If you use WP-CLI, which is a set of command-line tools for managing WordPress installations, the other option is to use wp media regenerate command. The functionality to generate image size on-the-fly is also planned to be included into core WordPress functionality: here is a branch in WordPress bug tracker where you can look up the status of this update.
- Edit the media setting of your theme again to its initial values (for TwentySixteen, 150x150 for thumbnails and 300x300 for medium-sized).
- Regenerate thumbnails' sizes for the first picture you used but keep the modified size for the second one.
When might you need to regenerate thumbnails for your website's media?
- After WordPress was updated
- After you have switched themes
- When there is a plugin conflict
- When your WordPress version becomes outdated
Answer 2. After you have switched themes
At which point are the additional sizes created for the pictures in your media gallery?
- At the moment the files are uploaded to the media library
- At the moment the picture is inserted to the post
- At the moment a new theme is activated
- At the moment user requests for this picture in specific size
Answer 1. At the moment the files are uploaded to the media library
Which image sizes are regenerated when using Regenerate Thumbnails plugin?
- Thumbnails only
- Thumbnails and medium-sized
- All of the additional image sizes generated by WordPress
Answer 3. All of the additional image sizes generated by WordPress