There's this generic image:
A tank with it's turret turned(Remove all the background ground and foliage, pretend it's just the tank with its turret pointed to the side):
One tank shooting at another(Remove all background ground and sky, pretend it's one tank icon blasting another)
Two tanks facing off with no firing(Remove all background ground and sky and stuff)
Just a tank firing(Remove all background ground and sky)
Using the one tank firing at another example your icon would appear something like:
One tank 'rushing' towards another tank, although this might be confused with movement:
