Open your Product Catalog and select the product you want to edit.

Select any of the images in the product listing.
Photoroom opens all of the product's images in the Product Catalog Editor.​

Select Resize.

Choose a preset size or select Custom size and enter a Width and Height.
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Photoroom resizes all of the images for the product listing.
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[Optional] Make additional changes using the Product Catalog Editor.
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After resizing, it is often useful to fine-tune the Position settings and also change the Backgrounds to fit the new size.
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When you have finished editing, publish your changes to Shopify.