"I'm amazed that he can paint such a piece" implies that he is still able to.
"Can" cannot be used if the artist is dead or has lost his mind or hands. It can only be used if the artist is alive and still able to paint well. However, in contrast, "could" does not imply that the artist is dead or has lost his ability.
If the artist were expected to paint more pieces of a similar standard, we might say "I'm amazed that he can paint such pieces" - can and the plural both become more likely in this case.
However, even if we expected the artist to produce more, we would probably still be more likely to say "I'm amazed that he could paint such a piece" when referring to a particular painting.
There is often a tendency to use past-tense forms when describing past events, even when the relevant details must still apply. For example, suppose I hear a story about a friend of mine who fell out of a boat and swam to the shore. I might say "I'm surprised he knows how to swim", or I might say "I'm surprised he knew how to swim" - with no implication that he will have forgotten how.