Relationships are more important than the law
Important first question: Is the painting worth fighting about? That is, does it have any significant monetary or sentimental value for the beneficiaries? If not, let the painter have it and avoid the fight.
Common law regarding transfer of personal property
Prima facie the painter was the original owner of the painting by virtue of creating it. At common law there are 6 ways that ownership of personal property can be transferred (and several involuntary ways that are not relevant):
- Purchase and sale (either of the painting itself or of the painting service as a work for hire)
- Transfer or gift by delivery
- Transfer by deed of assignment
- Transfer by declaration of a trust
- Inheritance under a will
- Taking possession of abandoned property
The most relevant to the facts is No 2: Transfer or gift by delivery. For this to have occurred the painting must have been given to the deceased with:
- An intention (at the time) to transfer title
- Delivery
- Acceptance of the delivery.
Delivery and acceptance cannot be denied as it has been hanging in the deceased's house for 8 years (unless the painter co-habited with the deceased; delivery has not taken place in that case). The painter has the onus to prove that they did not intend at the time to transfer title. It is difficult to see what evidence they could put forward that would convince a court of this if the facts are as you say.
Case and statute law in your jurisdiction may have changed the common law.
TL;DR
The Estate owns the painting.