The defence had said Jufer was intoxicated during the act.
The case has cast a rare spotlight on Thailand's lese majeste laws, which have remained virtually unchanged since the country's first criminal code was passed in 1908 despite the overthrow of an absolute monarchy in 1932.
Jufer was caught on surveillance camera
defacing portraits of the king [EPA]
Bhumibol, who is greatly loved by Thais and regarded by some as semi-divine, is protected from reproach by strict laws that forbid any criticism of the monarch.
The vandalism coincided with his 79th birthday, which was celebrated across Thailand with fireworks and prayers.
Jufer's trial was held this month behind closed doors to minimise publicity over the case.
During the hearing, one of the lead prosecutors asked journalists waiting outside the courthouse to leave, saying that it was not necessary for Thais to know about the case.
Jufer has a month to file an appeal.