на всю Jpanel ?

int width = 100;
int height = 100;

// Create buffered image that does not support transparency
BufferedImage bimage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

// Create a buffered image that supports transparency
bimage = new BufferedImage(width, height, BufferedImage.TYPE_INT_ARGB);

These examples create buffered images that are compatible with the screen:

GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
GraphicsDevice gs = ge.getDefaultScreenDevice();
GraphicsConfiguration gc = gs.getDefaultConfiguration();

// Create an image that does not support transparency
bimage = gc.createCompatibleImage(width, height, Transparency.OPAQUE);

// Create an image that supports transparent pixels
bimage = gc.createCompatibleImage(width, height, Transparency.BITMASK);

// Create an image that supports arbitrary levels of transparency
bimage = gc.createCompatibleImage(width, height, Transparency.TRANSLUCENT);

A screen compatible buffered image can also be created from a graphics context:

public void paint(Graphics g) {
Graphics2D g2d = (Graphics2D)g;
int width = 100;
int height = 100;

// Create an image that does not support transparency
BufferedImage bimage = g2d.getDeviceConfiguration().createCompatibleImage(
width, height, Transparency.OPAQUE);

// Create an image that supports transparent pixels
bimage = g2d.getDeviceConfiguration().createCompatibleImage(
width, height, Transparency.BITMASK);

// Create an image that supports arbitrary levels of transparency
bimage = g2d.getDeviceConfiguration().createCompatibleImage(
width, height, Transparency.TRANSLUCENT);
}