Though Christ is leaving the Earth to take His rightful place at the right hand of God the Father, He is still the source of blessings and wisdom, as they are sent down to the waiting Church (signified by His mother and the apostles). He is, after all, sitting on what appears to be a bridge. Christ is our bridge to Heaven - He is the only path which leads to salvation.
Speaking of the apostles, they're a jumbled mess. Some critics have said that this is because they are confused and scared. Others have said it was the artist's way of conveying intense grief. I honestly think it's a bit of everything. The apostles were surely grief-stricken at the thought of saying goodbye to their Friend and God again. No doubt His Blessed Mother was besides herself once again as she understood her place was to stay behind to nurture the growing Church.
Her beautiful hands are lifted in supplication to her now glorified Son. She does not look mournful, but she does look expectant. She understands that while Heaven awaits her Son, earth would now await His Return, and she would be first in line!
So while the apostles around her held their heads in grief, or held one another in mid-gasp, Our Lady was already looking forward to the day she would reunite with her Son (whether through His 2nd Coming, or her own passage into Heaven - who knows if she knew which it'd be).
Our Lady is also in a deep crimson red. Red is the color of power - of the Holy Spirit. The Blessed Mother is, after all, "full of grace" and as a result is the most powerful intercessor of all. Flanking her on either side, then, must be Saints Peter and Paul. Though Saint Paul was not present at the actual Ascension (having not had his conversion), the artist probably chose to include him anyway so as to speak of the gentiles the Church would come to encompass.
One final note about Our Lady. She is standing on what appears to be a little square platform. I thought about this for a bit, and I assume it's meant to symbolize that though Mary was left in the world, she was not of it. She is there, but she's above it - closer in union to Her Son through her holiness. There's a story from her childhood in which her mother, St. Anne, wouldn't allow the Blessed Virgin's feet to touch the floor until she was three years old (some cultures still have this practice). The earth is impure, so to have Our Lady standing above it a bit... I wonder if that's significant of her holiness and grace.
Anyway, it's a beautiful image celebrating a beautiful day. Today, 40 days after Easter, is when Christ once again reunited with His Father in Heaven. We celebrate not only His glorious ascension, but the fact that He will come again... in the same glory He ascended with!
Blessed be, blessed be, blessed be. :)