Go to Theme settings -> Product Page
+ Scroll down to "Upload image size chart"
Choose the image and enable "Show 'Size chart' img?"
+ Besides, you can use Metafield Editor app to add the different images size chart for one by one product.
Please check this link: https://wiki.the4.co/blogs/gecko-wiki/how-to-use-metafields-editor-app-in-gecko
+ After, scroll down to "Product Features" -> Enter the name of the variants
*** Image size chart
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