A painting is a made up representation of a person.
A photograph is much more a definitive likeness.
Most biographies are not books about the facts of a person's life, although there are definitely facts in there. They are more about why he did this and what was he thinking at such and such a time, and pseudo amateur psychologist/authors sort through his life and say shit like, "well because his mother did this to him when he was 3.... then when he was 30 he did that..." and other such stuff.
"his early bed wetting was the reason he later invaded Russia...."
"he had a mommy complex, so in later years he dressed like her while doing the ironing..."
"he became secretary of state because he had a thing for his dads secretary when he was 9...."
" the screen of the iPhone, iPad, and iPod is black because he suffered through bouts of depression"...
That stuff sounds like a painting, not a photograph.