The picture is from a manuscript that was written in the 14th century.
The painting on its bottom says:
"цртвовасилиамакедωнянинa"or "црcтво василиа македωнянинаc" and translates: " Empire of the King the Macedonian"
Boris was baptized in the palace in Pliska, Bulgaria, in 864 AD. Pliska was the capital of Danubian Bulgaria between 681 and 893 AD. Pliska was founded in 681 by Khan
Asparoukh. Boris I Michail (he was son of Khan Presijan of Bulgaria) was the Khan from 852 to 889 and first Christian ruler of Bulgaria. Michael became part of his
name (i.e. his adopted "Christian" name) after his baptism in 864.He was baptized during the reighn of the Byzantine Emperor Michael III.
oldest dated Slavic text is a gravestone inscription erected by Tsar Samuel of Macedonia in 993