The first problem is that destruction stages are triggered as the 'health' of the object decreases, and as far as I can tell the health is damaged by
any weapon; in other words, you can't restrict it to damage by particular weapons as you want. You
can check for an object being hit by particular weapons in a script, using the OnHitWith block. If you set up a form list that contains the list of weapons you want, you can use that in the block command ( e.g. BEGIN OnHitWith FlameWeapons). Giving karma to the player is as simple as including the command 'player.rewardkarma x' (where x is the karma value) in your script.
To do all this, of course, you would have to change the posters from simple statics to something that can take a script and/or destruction data, such as an activator, and then go through the game replacing all the existing posters with yours. In addition, to use the OnHitWith block the object has to have a collision surface, which the current posters do not. That might also be true for destruction stages to work, but I don't know for certain.