What is it called when someone is utterly absorbed in a piece of art?

A couple ideas come to mind. This one can be a bit difficult since being absorbed in something seems to require focus, yet it also seems involuntary.

The first word that came to my mind is transfixed.
Transfix is a transitive verb defined by Merriam-Webster as

: to hold motionless by or as if by piercing

This one makes a good bit of sense as its second definition deals with being physically impaled. The art, in this case, is almost forcing you to stop. However, a drawback to this word is that it does not imply much emotion.

Another word that came to mind is entranced.
Entrance is also a transitive verb. It is defined as

: to put into a trance

This one has a bit less force to it, but it more accurately describes that sensation in an emotional sense.

Similar to entranced, you could use enthralled.
Enthrall is another transitive verb. It is defined as

: to hold spellbound : Charm

This one is very similar to entrance.

A fourth word worth thinking about is selah.
Selah is a Hebrew word found throughout the Bible in the book of Psalms. It is generally understood by the context to be a sort of musical direction. Here's an example:

1 O LORD, how many are my foes! Many are rising against me;
2 many are saying of my soul, there is no salvation for him in God. Selah
3 But you, O LORD, are a shield about me, my glory, and the lifter of my head.
4 I cried aloud to the LORD, and he answered me from his holy hill. Selah

Nobody actually knows the true meaning of the word; however, many suggest that it means something like "stop and listen". This could be something related to that sense of being absorbed in something. Here is the link to the Wikipedia article for the word: https://en.wikipedia.org/wiki/Selah

I hope this answered your question!

I'd suggest the word rapt. It means, "carried away in an ecstatic trance" from Latin raptus, past participle of rapere "seize, carry off". A figurative sense, the notion is of "carried up into Heaven (bodily or in a dream)".

Oxford dictionary defines it as,

Completely fascinated or absorbed by what one is seeing or hearing.

The children sat rapt as the puppets danced.

The old guy was seeing the painting with the rapt attention.