TLDR: None worth academic interest.
All modern encryption techniques intended for digital computers are applicable, and secure, for all kinds of digitized data, including text and image. There is no need for such encryption specialized to image or text. Argument: the modern baseline for encryption security is resisting Chosen Plaintext Attack, and that model assumes the attacker can choose the plaintext, including image in whatever format.
We can encrypt image, or pretty must anything digital, with e.g. AES-CTR or Chacha. These are widely available on most computer platforms, there is no known attack against them (baring side-channel attacks and IV reuse, which really are attacks on the implementation), and with a competent implementation, speed is faster than most processing peripheral to image processing, like compression, transmission, storage.
From a sampling of papers on image encryption (those at MDPI are free access), most have precisely no advantage over standard techniques: these paper's technique typically is slower and comes with weak security claims (often, relying on some standardized algorithm or metric; that's even though, assuming the existence of secure encryption, we demonstrably can't make a polynomial-time algorithm that recognizes good from bad encryption by examining ciphertext and plaintext). Reasons for why these papers are published (often, for a fee) are discussed there. Whatever these reasons are, peer-reviewed IACR publications seldom if ever buy them: I've only found preprints with this query.
Still, there are a few legitimate reasons to special-case image:
- It calls for special techniques for the most efficient data compression (before encryption), in order to exploit the correlation between nearby pixels and characteristics of the human eye. There is an interaction with encryption, in that we must be careful that the compressed size does not leak exploitable information.
- When we want the ciphertext to be a meaningful image: steganography, and Thumbnail-Preserving Encryption (where plaintext and ciphertext are images, and the ciphertext a thumbnail of the plaintext).
- When we want decryption by analog means, such a superposition of transparencies. That's visual cryptography, a legitimate (if limited) field of research.
- If we also wanted encryption by analog means. That's the goal of optical image encryption. It has been theorized in this (extremely cited) article (1995), and demonstrated experimentally (1996). I find several reports that it is practically broken in a ciphertext-only setup believable. There are countless variations, many simulations only with bogus security argument.
