What do you want this Founder's portrait art to look like? Please list the physical description, Action & Magic, Clothing, Accessories, and Background that you would like to see for Tremere's portrait.
Pablo Moreno did a very nice series of the Founders, for the Spanish book, I believe?
While not all of them are perfect, the Tremere one is pretty good as far as I'm concerned.
And please remember, Tremere was the youngest of the Founders, which was (likely) the cause of much of his insecurity.
Whilst this is meant to be a portrait of Tremere, his followers are fundamental to his character, so I'd include some representations of them as well.
A young italian man wearing blue-black robes, with a "Mars enscribed with a square" cloakpin. He has slightly wolfish features. At his waist he wears a walnut wand inlaid with gold and lead. It's clearly meant to be impressive, but ends up rather gaudy. In the background is a land of mountains and forests (you could use a specific view of the Carpathian mountains). He is engaged in the certamen duel that has taken the form of a ghostly chessboard (Rego Mentem) against an opponent who is off the side of the picture.
Watching the duel (and clearly supporting him, but very much not the main focus of the picture) are a small group of Dacian necromancers (eastern europeans, evidence of arcane connections to the dead).
In the background of the scene is a small pack of white wolves. The leader of the pack has some features that are slightly reminiscient of Tremere (e.g. same eyet colour which is different from the rest of the pack.