Yeah, this paper is a good representation of the whole "chaos cryptography" theme — snake oil cryptography invented by authors who don't even attempt to study cryptography before producing their bullshit and publishing it in sketchy or generalist, unrelated to cryptography, journals.
In this particular paper:
1. The motivation for "image encryption" is completely wrong, as you pointed out.
2. There is no secret key. The "key" is produced by hashing the image with SHA-256.
3. Claim: "No any useful information can be acquired from encrypted images". Actually: since this is convergent encryption (https://en.wikipedia.org/wiki/Convergent_encryption, "key" derived from SHA-256 of data), encrypting any two equal images will produce the same ciphertext, globally.
4. Finally, the actual "key" can't be more than 64 bits, since SHA-256(image) is then split into two groups of 32-bit words, which are xored together to produce two key values. Thus, this whole "encryption" scheme is limited to 64-bit security (probably even less than that). We don't even have to analyze the whole "chaos" system they invented to conclude that it's insecure.