You think they are shadows extending into the carpal area from the protruding tip of the ulna?
The left side of the tip of the ulna is clearly brighter than the right side. There appears to be some very slight discoloration in the carpal area, but it appears much less than where the finger bruises appear. I have a hard time calling it a bruise, at least from the photographs that I am looking at.
If we could see good quality original photos, from a couple of different angles, we could know much more.
I hope the police took photos, but so much time had elapsed, and no witness has come forward (that I know of) who saw the bruising prior to her taking the selfie, that it likely does not matter. It seems to come down to a “he said, he said, as Fields herself did not identify Lewandowski, as far as we know. She was virtually face to face with him in the video; perhaps she IDed him from pictures.