Can the word "masterful" be applied to a work of art, to compliment the perfect technique of the artist? (In the dictionaries, the word has also some negative connotations)

why not? this is yet another example where one could quote ol' Humpty Dumpty.
-joe (meaning exactly what I mean) friday

Works for me. Try googling "masterful portrait" or "masterful watercolor" and see what you get. See A-H (link).

jheem, I get a 999 error from that link.

Works for me.

seems a masterly link here.

MWCD comments: Some commentators insist that use of masterful should be limited to sense 1 in order to preserve a distinction between it and masterly. The distinction is a modern one, excogitated by a 20th century pundit in disregard of the history of the word. Both words developed in a parallel manner but the earlier sense of masterly, equivalent to masterful 1, dropped out of use. Since masterly had but one sense, the pundit opined that it would be tidy if masterful were likewise limited to one sense and he forthwith condemned use of masterful 2 as an error. Sense 2 of masterful, which is slightly older than the sense of masterly intended to replace it, has continued in reputable use all along; it cannot rationally be called an error. [see here for senses 1 and 2]

To compare it with artful:
ADJECTIVE:

1.Exhibiting art or skill: "The furniture is an artful blend of antiques and reproductions" (Michael W. Robbins).
2.Skillful in accomplishing a purpose, especially by the use of cunning or craft.
3.Artificial.

Here negative meaning came in by degree.

Thank you for unearthing this interesting history !

Here negative meaning came in by degree - e.g. Dickens' Artful Dodger..

Tidy?! Tidy?!?! We are not talking about "tidy" here, m'sieur! We are talking about language!

Indeed, "tidy" exists as a part of the language, but its attributes do not obtain to language.

Exactly. This is just the same reasoning that gave us that split infinitive monstrosity. There was never a prohibition against splitting infinitives in English, either in normal speech or in the writing of good writers. Then some guy came along and said he didn't think it was very tidy and he didn't like it and suddenly every rabid prescriptivist is saying anyone who splits infinitives is an ignorant lout who should be shunned by all right-thinking individuals. The good folk at Language Log refer to these things as "zombie rules" because of their ability to keep going no matter how many stakes are driven through their hearts.