Scholars go by Islamic principles when issuing such fatwas. If the pictures:
1- do not desecrate the Imams
2- do not lessen their respect in society
3- do not make people think that this is how the Imams actually looked
Then by default Islamic law it would be permissible. But if the pictures reduce from the sanctity of the Imams in any way, then they would be haram. This could differ from one society to another, as different societies have different standards and perceptions.