In Magento, you may need to regenerate thumbnails if you’ve changed the size of product images or if the images are not displaying properly. Here are the steps to regenerate thumbnails in Magento:
Log into the Magento admin panel.
Go to System > Configuration > Advanced > System > Image Processing Settings.
Change the “Adapter” setting to “GD2” and then click on the “Save Config” button.
Go to System > Index Management and select all indexes, select “Reindex Data” from the Actions dropdown, and then click on the “Submit” button.
Go to System > Cache Management, select all caches, select “Refresh” from the actions dropdown, and then click on the “Submit” button.
Go to System > Configuration > Catalog > Product Image and set the image sizes for small, thumbnail, and base image.
Go to Catalog > Manage Products and select the products you want to regenerate thumbnails for.
In the “Actions” dropdown, select “Update Attributes” and then click on the “Submit” button.
Check the front-end of your site to ensure that the thumbnails are displaying correctly.
Note: If you’re using Magento 2, you should check the documentation for more details on how to regenerate thumbnails.
Also, If you have a large number of products, you may consider using an extension to automate the process of thumbnail regeneration.
