AssetImage _imageToShow;

@override
iniState(){
   _imageToShow = AssetImage('youAssetImage');
}

Container(
            padding: EdgeInsets.only(left: 10.0),
            height: MediaQuery.of(context).size.height * 0.5,
            decoration: new BoxDecoration(
              image: new DecorationImage(
                image: _ImageToShow,
                fit: BoxFit.cover,
              ),
            )),
 ),

void updateImage() {
   setState ((){
      if (someCondition) {
        _imageToShow = new AssetImage(lesson.imagePath);
      }
    });
}