what is the term for the act of assigning unique identifying values to digital images using blockchain technology?

The most widely used term for this is “minting an NFT (non-fungible token)” for the image.

More precisely:

• An NFT is a unique digital identifier recorded on a blockchain that certifies ownership and authenticity of a digital asset such as an image.

• When you “mint” an image as an NFT, you are assigning it a unique on‑chain identifier (the token ID plus the contract address) that links to that image and its metadata.

• Technically, this relies on cryptographic hashing of the file or its metadata, and the hash (or a pointer to it) gets stored in a blockchain record, but in mainstream usage the overall act is called NFT minting rather than “hashing an image”.

So, in everyday blockchain/crypto language, the term you’re looking for is:

Minting an NFT of the image (or simply NFT minting), which creates a unique blockchain-based identifier tied to that digital image.