You are correct in your observations. When RadImageGallery is bound to a folder or a data source containing binary data the images are dynamically created and there is no way to know the big image URL before creating it. This is because the items are created on demand only when required in order to optimize performance.
However, if you want to access the big image URL the approach you could implement is to create a static collection of RadImageGallery items and set its ImageUrl property on the first page load. This could be achieved by manually converting the bytes into files and then providing the URL's to the new files.
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