"Les portraits de Grégoire l'Illuminateur dans l'art byzantin," by Sirarpie Der Nersessian, from the journal Byzantion, Vol. 36, No. 2 (1966), pp. 386-395, in 16 searchable pdf pages, with plates. The founder of Armenia's Apostolic church also is a saint in the Eastern Orthodox and Roman Catholic churches, and is represented in their sacred art. A Wikipedia entry describes Gregory's life and deeds. Attached to the document are links to additional resources. Internet Archive has other works by the author, who was a renowned art historian.