The movie RocknRolla has strong characters and the "feel" to it but to me it seems to have a convoluted plot and loose ends. But this one bummed me specially. Why does Cookie give the painting to "one two"? I did not get the connection.
Because One Two told Cookie about the date in the museum or because One Two told anybody else about the date in the museum and anybody else told Cookie about it.
Going to a contemporary art museum is not something a regular person would do. Only a "cultured" person would do that as Tank puts it.
Therefore, the accountant is a cultured person.
Any work of art is a very thoughtful gift for her.
Cookie is simply helping One Two with his romantic gestures.