I don’t understand why if you already have the image? You would have had to uploaded it to get it into the CMS. If what you mean is you want the visitor to “download” the photo you could just link directly to the asset in custom code. So you have an image element you wrap with a link that points to the URL.
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If you don’t use a collection list for the images then you would have to manually insert a valid URL to each one into a custom code embed.