When uploading product pictures, upload the largest size you want to use. For product pictures, you'll want to put the pictures used for zooming. You can then set the image sizes throughout the theme editor. The high quality, built-in resizer will resize images the first time requested. It then caches them for future use.
Using a separate thumbnail picture is optional and only necessary if it is different than the larger image. The regular image is used on the product details page and the zoom pop-up pages. The thumbnail image is used on the product list pages, widgets and the image thumbnail array on the product detail page.
The resizer settings are under the Page-Specific Settings section in the Theme editor.
To access the product picture settings for the zoom window, select your theme under the Themes menu in the administrative console. Click on Product Picture (Popup) on the left side under Pages.
You can make several picture settings including width and height. There are also picture setting under the Category, Product Details and Shopping Cart under the Pages section of the Theme editor.
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