From what little we can see, the suit she's wearing seems sleek and stylish in all black with the feathers on the shoulders clearly indicating who is wearing the suit (thank you, Cinna!). After taking another look at the teaser trailer, it doesn't seem like Katniss is wearing the Mockingjay suit, could be a specific reason for that? Also, it seems important to note that her arrows in this poster aren't color-coded like they are in the book. Could this mean something?
Hopefully, we get more answers on September 10, based on this announcement on the Our Leader The Mockingjay app:
Tiffany created a wallpaper using the poster. Feel free to use, dimensions are 2880x1880 (click to enlarge).