My advice is to follow the steps in the article. If it is a print of this painting, it may well have a low resale value, particularly if it is a fairly modern reproduction. Google the name of the painting followed by the word 'images' to see if it is either a copy or a reproduction of Chagall's original work. Also look for gallery labels on the reverse of your item which may indicate if it is a print. If the label is from a major museum, then it is unlikely to be an original.