Dear Gordo,
I agree that the most complete icon of Pentecost is WITH the Most Holy Theotokos!
However, there are those that do not represent her and they are perfectly valid icons of Pentecost.
The fact that those icons do not depict the Mother of God despite the fact that Scripture states plainly that she was with the Apostles in steadfast prayer until Pentecost need not detract from the icon's worth or validity.
For example, icons of Pentecost, as the one you show, depict, at the top, St Paul the Apostle sitting with St Peter.
Now we know for a fact that St Paul had not yet come on the scene!
But he is depicted as one of the two "Chief Apostles" who later shared as well in the fullness of Pentecostal Grace and the mission of the Apostles of Christ.
An icon ultimately is not necessarily obliged to depict all the facts associated with an event - indeed, it will bring it elements and symbols that transcend the event, even though they are related closely to it.
The figure below, at the base, is sometimes referred to as a symbol of the "cosmos."
In fact, it is the Prophet Micah in whose book we find the words about the outpouring of the Spirit so that "young men would see visions" etc.
Alex