Some objects can display pictures in game, using the command setObjecttexture. It's a property of the addon/model. Back in the OFP-days many soldier addons had armpatches that could be changed this way.
In vanilla ArmA2 you can use setObjecttexture on the flags and the video screen, found under Empty >> Objects.
The image should be in .paa format and goes directly into the mission folder (or any subfolder of your choice there in)