JUnit test for Custom image class

I have this custom image class called ColorImage:

public class ColorImage extends BufferedImage
{
/**
 * Create a ColorImage copied from a BufferedImage
 * Convert to 24-bit direct colour
 * @param image The image to copy
 */
public ColorImage(BufferedImage image)
{
    super(image.getWidth(), image.getHeight(), TYPE_INT_RGB);
    int width = image.getWidth();
    int height = image.getHeight();
    for (int y=0; y<height; y++)
        for (int x=0; x<width; x++)
            setRGB(x, y, image.getRGB(x,y));
}

/**
 * Create a ColorImage with specified size and 24-bit direct colour
 * @param width The width of the image
 * @param height The height of the image
 */
public ColorImage(int width, int height)
{
    super(width, height, TYPE_INT_RGB);
}

/**
 * Set a given pixel of this image to a specified color.
 * The color is represented as an (r,g,b) value.
 * @param x The x position of the pixel
 * @param y The y position of the pixel
 * @param col The color of the pixel
 */
public void setPixel(int x, int y, Color col)
{
    int pixel = col.getRGB();
    setRGB(x, y, pixel);
}

/**
 * Get the color value at a specified pixel position
 * @param x The x position of the pixel
 * @param y The y position of the pixel
 * @return The color of the pixel at the given position
 */
public Color getPixel(int x, int y)
{
    int pixel = getRGB(x, y);
    return new Color(pixel);
}
}

So I need to write JUnit tests for this class, I have done some JUnit testing, but never before have I tried to write unit test for image classes. I understand this is very messy and I don't have my version of unit test attempts, its just that I have a lot of assignments to do, so time is very limited. Any help would be greatly appreciated. Thanks