Let us remember that motifs are defined as recurring structures, devices and contrasts that can help develop and inform the major themes of the text being studied. Clearly, one central motif to this novel which is inescapable is that of the portrait of Dorian Gray. Note that it is referred to as "the most magical of mirrors" and reflects his soul by indicating the physical consequences of his actions and which he is spared from suffering in his own body. This parallels Dorian's hedonistic quest to live life satisfying his own desires and seeking pleasure first and foremost by constantly coming to haunt him with guilt and conscience:
Eternal youth, infinite passion, pleasures subtle and secret, wild joys and wilder sins--he was to have all these things. The portrait was to bear the burden of his shame; that was all.
In it he sees echoes and traces of his sins. In particular the cruelty that characterised his relationship with Sibyl Vane is present there in the smirk that he sees and the blood on his hands after his murder of Basil.
As with any motif, you would benefit from tracing the portrait and how it operates through the novel, identifying its importance and how it relates to other key themes in the text.