Back then in India at the time of the Buddha there were many kingdoms. To maintain peace between them good relationships between the kings was crucial. A common way was to express good relation through gifts.
King Udrayana send to king Bimbisara — kingdom of which was much less powerful — an armour ornamented with precious stones.
Bimbisara felt preassure to repay the kindness of Udrayana, but didn’t know what to give, so he went to the Buddha, who lived in his kingdom. The Buddha comissioned then a painting to be given as gift, with highest quality materials.
Buddha told Bimbisara to deliver the gift in the grounds that would usually be battlefields, army included! So Udayana was confused and angry after the precious armour he gave. Udayana’s ministers told him it should be OK to go there to receive the “gift” in return, because Bimbisara was much less powerful.
Bimbisara obviously didn’t want to attack so when Udrayana saw him approaching with 2 ministers and then dismounting from their horses — with the object wrapped in expensive clothes — they got relieved.
Bringing the armies to this encounter was only to signify massive respect around the offering of the gift.
After seeing the painting Udrayana enquired about the figure on the top right of the painting: the Buddha. So now Bimbisara’s kingdom became very respected because through that painting Udrayana started analyzing, studying the painting, meeting the Buddha and following the path to liberation.
The Buddha thought this painting would be useful to have at monasteries so that monks could explain to pilgrims, since he anticipated the decline and degenerate times.