I was working on a new Woocommerce site and I wanted to change the thumbnails size after already uploading many images and I wanted to do the change in bulk. I found Regenerate Thumbnails, a plugin that allow you to regenerate all thumbnails, individual thumbnails or specific thumbnails for your WordPress image attachments. This is very handy if you’ve changed any of your thumbnail dimensions (via Settings -> Media or via your theme settings) after previously uploading images.
How to regenerate image thumbnails with Regenerate Thumbnails plugin:
- Install the plugin
- Change the image thumbnail sizes in your theme settings and/or WordPress media settings
- Go to Tools–>regenerate thumbnails and click regenerate thumbnails. It will regenerate thumbnails of all the images uploaded previously.

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Yeaahh cool.. it works successfully. thanks