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What is the URL for the zoomed image that we see for the items on eBay?
The zoomed in image, shown in the Zoom layer on ViewItem and in the Supersize popup window/layer can be accessed by replacing the _1 or _12 (returned in the FullURL) with _57.
If the Full-Image URL is http://i.ebayimg.com/00/s/MTYwMFgxNjAw/z/~1YAAOxy~g5Rt0bS/$T2eC16ZHJF0FFZ5Ddu6oBRt0bRm-bw~~60_12.JPG?set_id=880000500F
if you replace the _12 at the end with _57 i.e http://i.ebayimg.com/00/s/MTYwMFgxNjAw/z/~1YAAOxy~g5Rt0bS/$T2eC16ZHJF0FFZ5Ddu6oBRt0bRm-bw~~60_57.JPG?set_id=880000500F will give you the zoomed image.
However, it is it critically important that any Add/ReviseItem requests only ever use the _1 or _12 image in the PictureURL containers i.e the FullURL returned by the UploadSiteHosted API response. The use of any other member image in an attempt to show a larger image on ViewItem will result in the Gallery image (shown on SRP) not being generated. It is fine to use the _57 for viewing, and is the best choice if the you are importing the items and images to a local application.