First of all, I don't think the crosshairs were 'placed'. Given the angle, it looks like the crosshairs were part of the writing on the background.
Now, its possible that the creators of the image did intend to imply it as some sort of threat. But then, Stone wasn't the original creator of the image.
Its not the first time he has posted images without thinking... He once posted a picture of Himself, Trump and a bunch of other associates wearing space suits (in honor of Trump's plan to create "Space Force".) The problem is, the suits they were wearing had swastika on them.
https://www.newsweek.com/roger-stone-space-force-swastikas-1072213
Again, I'm not saying this to absolve Stone. From what we know, he definitely belongs in jail, for a long long time. And he should definitely have kept clear of posting ANYTHING about the judge or the case. I'm just not convinced the 'crosshair image' was actually posted as a threat by Stone aimed at the judge.
If you're being charged with witness tampering it's best to avoid anything that might be mistaken as threatening someone involved in your trial.