Creating an image with BufferedImage class

int width = 256; //in pixels
int height = 256; //in pixels
BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_4BYTE_ABGR);
//BufferedImage.TYPE_4BYTE_ABGR - store RGB color and visibility (alpha), see javadoc for more info

Graphics g = image.createGraphics();

//draw whatever you like, like you would in a drawComponent(Graphics g) method in an UI application
g.setColor(Color.RED);
g.fillRect(20, 30, 50, 50);

g.setColor(Color.BLUE);
g.drawOval(120, 120, 80, 40);

g.dispose(); //dispose graphics objects when they are no longer needed

//now image has programmatically generated content, you can use it in graphics.drawImage() to draw it somewhere else
//or just simply save it to a file
ImageIO.write(image, "png", new File("myimage.png"));