I heard the following story from a friend today. Somebody once wanted to photograph the Rogatchover Gaon (horav Yosef Rosin) zt"l but was refusing to have his photo taken.
Rabbi Meir Shapira zt"l, present at the time, turned to the Rogatchover and said: "When Yosef Hatzaddik was in Mitzrayim he was able to draw strength from visualising the image of his father Yaacov Avinu in his mind.
Perhaps there will be someone who will see the image of a Tzaddik and having that image in his mind will at some point inspire him to make a right choice in his life.."
Considering this prospect, the Rogatchover agreed to have his photo taken.
When I paint a portrait of a Tzaddik, I spend many hours looking closely and carefully at the features and expressions of that holy person. It brings that Gadol alive in my head, it inspires me and makes me aspire to learn something from that persons life.
Here's a painting of the Chasam Sofer (Rabbi Moshe Sofer zt"l) that I just finished.
Here is a quick sketch I made of the Chasam Sofer; a copy of a drawing by Josef Kriehuber (about 1830). Note the difference in his facial appearance.
In the painting I was trying to combine the information from the drawing mentioned and from a painting of the Chasam Sofer by Josef Edward August von Gillern (about 1835). I Also drew him younger then he was when either of the portraits were made.