Move your image initialization/declaration into the init method. If you wish to call the image in the render scope, you should have the image set to be a global variable, then initialize it in the Init method.
Edit: also, image classes in slick have draw(x, y) methods available! So no need for g.drawImage(). Example: Image img = New Image("res/Image.png");
Render method: img.draw(0, 0);