A little info on paintings of thangkas of lamas together with their previous lives: it is a very powerful tool to generate the merits for the particular Lama to return, if the Lama has passed away, and to resume his Dharma work in full force. Every time his disciples make a prayer to this thangka, they create the cause for their Lama to return and for the Lama's Dharma work to expand and grow. Now, imagine the merits that will be created by the person who painted this thangka so that others can pray to it.
Correct me if i am wrong, but if i remember, thangkas of these type are call trungkap