Someone had asked a similar question some time back. See the answer here, with examples:
The consensus seems to be that showing products in use, leads to higher conversion rates.
And for T-shirts, having a relatable human wearing it always helps. There is a theory of visual communication that holds, a human face is the most interesting thing to another human.
But as with all things related to conversion, test, test, test, If you don't have a lot of traffic test intermediate goals like click through on the images or add to carts. If you have enough traffic, test actual sales.