The square with the circle represents the building, in the centre of which lives deity. The building is constructed to face in four directions. According to Tibetan convention, east is in front, and south, west, and north follow in the clockwise direction. There is a door in each direction, generally guarded by gate protectors. Here each gateway has a Dharma chakra. The square inside the doors has auspicious stylized symbols and designs.
The inner square contains nine smaller squares containing auspicious symbols and syllable mantra Ah, encircled with lotus petals, that represent deities. Ah is the vijamantra of Amoghasiddhi Buddha. Amoghasiddhi is the head of the action Buddha clan, and he symbolizes the practical realization of all the wisdom of the other four Tathagatas. Moreover he is believed to be 'unfailingly successful and to have the power of infallible magic.
Outside the circle, upper and lower corners are rendered with syllable mantra Ah encircled with lotus petals. The background of the painting is black which has a special meaning in esoteric practices and rituals. Moreover black background and black thangkas are considered as highly mystical and esoteric, usually reserved for advanced practices. It is used for terrific ritual actions, the radical conquest of evil in its forms conquests not by annihilating, but by turning even evil into good. Further the present thangka is very suitable for the sadhana to tame the bad elements and motivations and for boon of peace and prosperity.
This description by Dr. Shailendra K. Verma, his Ph.D. thesis being "Emergence and Evolution of the Buddha Image (From its inception to 8th century A.D.)."