Blob Storage is the perfect place for images on Azure.
CDN is great when your images are at least somewhat static and will be viewed more than once.
With CDN, you do not pay for storage costs (on CDN), but instead for transfers from CDN - http://azure.microsoft.com/en-us/pricing/details/cdn/
It should also be mentioned that if you use Azure and Blob storage, this is the main means for storing images, the Azure CDN will not necessarily be your main CDN - and any CDN will work. CDN is currently a good product service.
NTN
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