According to TEP #64 (Token Data Standard):

NFT metadata attributes

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4. image - Optional. ASCII string. A URI pointing to a resource with mime type image.
...

Uniform Resource Identifier (URI) are not bound to the http protocol and you can use any protocol.

Be aware that while you can use any protocol you want, anything that will fetch your 'image' attribute from your NFT metadata will have to know how to deal with this protocol.

Keep in mind that I've written this answer by searching information and not testing.