St. Dominic Worshipping the Trinity
Baptismal Chapel, Cáceres Cathedral, Cáceres, Spain
The audio guide provided by the cathedral identifies the saint as Dominic, but there are no attributes other than his Dominican habit. Placing Dominic before the Trinity specifically may reflect his preaching against Albigensianism, which holds that the Son and Holy Spirit are mere creatures of the Father (Catholic Encyclopedia, s.v. "Albigenses").
In other Trinity images of this type the Son holds a large cross. Here the right hand is broken off and could have once held a cross.
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More of St. Dominic
More of the Trinity
Photographed at the cathedral by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.