Got it down to a single file (ok I cheated with one inner class), a single line "test.getGraphics()" demonstrates the effect. Not sure if the state based game stuff plays a factor but I used that here.
If you drop it into a package 'slick' within TWLExamples/src, and add slick.jar to the build path for that project, etc., you shouldn't have trouble getting it compiling.
Set the boolean at the top "getGraphicsTest" to true and it'll call Image.getGraphics(), and you should see the glitch (I hope). It uses login.xml for a theme.

Code:
package slick;
import java.net.URL;
import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.GameContainer;
import org.newdawn.slick.Graphics;
import org.newdawn.slick.Image;
import org.newdawn.slick.SlickException;
import org.newdawn.slick.state.StateBasedGame;
import slick.twl.BasicTWLGameState;
import slick.twl.TWLStateBasedGame;
import de.matthiasmann.twl.Button;
public class SlickTest extends TWLStateBasedGame {
private boolean getGraphicsTest = false;
public static void main(String[] args) {
try {
AppGameContainer container = new AppGameContainer(new SlickTest("SlickTest"));
container.setMinimumLogicUpdateInterval(20);
container.setMaximumLogicUpdateInterval(20);
container.setDisplayMode(1440, 900, false);
container.setShowFPS(false);
container.start();
} catch (SlickException e) {
e.printStackTrace();
}
}
protected SlickTest(String name) {
super(name);
}
@Override
protected URL getThemeURL() {
return SlickTest.class.getResource("../login/login.xml");
}
@Override
public void initStatesList(GameContainer container) throws SlickException {
addState(new TestState());
}
private class TestState extends BasicTWLGameState {
private Button btn;
@Override
protected void createRootPane() {
super.createRootPane();
rootPane.setTheme("login-panel");
btn = new Button("Exit");
btn.addCallback(new Runnable() {
public void run() {
System.exit(0);
}
});
rootPane.add(btn);
}
@Override
protected void layoutRootPane() {
btn.adjustSize();
btn.setPosition(20, 25);
}
@Override
public void init(GameContainer container, StateBasedGame game) throws SlickException {
Image test = new Image(1400, 800);
if (getGraphicsTest) {
test.getGraphics();
}
}
@Override
public void render(GameContainer container, StateBasedGame game, Graphics g) throws SlickException {
}
@Override
public void update(GameContainer container, StateBasedGame game, int delta) throws SlickException {
}
@Override
public int getID() {
return 0;
}
}
}
Hope this helps find the issue, I've been dying for a good theme-able Java GUI library (that isn't Swing) for years!