1] Screw cinematic fires, they are ugly. Go to buffs in models section, selection humans building damage fire, they are more adequate. Maybe even use Immolation Damage from Pit Lord.
2] Learn to scale trees randomly in X-Y and Z axis independently! There's two friggin buttons for that, and it makes it look so much better.
3] I think that grass is fine, but import is not required - go to River Rushes, select biggest model, colour it suitably. I myself make a few objects of different sizes (small-medium-large) and a second set of slightly different colour (like red and reddish orange).
4] Save the pics as .jpg file. If you do your printscreen through paint - screw paint, either find Photoshop (we all know where and how) or GIMP (that one's free and is said to be as powerful and versatile)