You can change the poster frame in different ways. Go to the File Info menu and select the General sub-menu.
Just park on the frame you would like to use a poster frame, and then hit the "Grab" button.
You can also upload a picture, select the "Upload" button, and hit the reset if you would like to return to default. By default, ioGates automatically grab the poster 33% into the movie, but you can change this from the Preference "Preset" menu. It can be good for dailies to set the poster grab to around 75 frames as then, there is a better chance to grab the clapper board.
View of the Preferences menu poster grab setting.