What would you call a picture where all the main characters are together?

Solution 1:

You can consider ensemble poster as well. These pictures are usually posters and usually includes the ensemble photography of the cast.

An ensemble cast is made up of cast members in which the principal actors and performers are assigned roughly equal amounts of importance and screen time in a dramatic production.

From Friends:

Series creator David Crane wanted all six actors to be equally prominent, and the series was lauded as being "the first true 'ensemble' show".

There are different styles of ensemble posters also. You can check this link: http://www.empireonline.com/features/ensemble-movie-poster-styles/11.asp

Solution 2:

It’s just a cast shot.

If you want something fancier, you might be able to wedge in something like tableau, but that’s a bit pretentious.