Screen shots are not accepted at most venues but I know for a fact that the TD Garden in Boston will not accept a screen shot. Unfortunately you will have to call Stubhub and wait on hold to talk to them.
I live in the area and regularly attend events at TD Garden. I don't care what Stubhub is telling you but I don't think a screen shot will get you in. I suggest that you call TD Garden customer service and ask them.
Also this link pretty much confirms that a screen shot will not get you in. So sorry to say that you will have to continue to fight Stubhub on this.