Right leg in a brace, a young boy lies on the couch while using his cellphone.
The right leg is in a brace
Only one leg is described, so 'the' is appropriate; since it is described as in a brace, it is in a brace.
Right leg in a brace is a paraphrase of right leg is in a brace.
If his right leg is in a brace, then his leg is in a brace.