Answer is already available on stack, still I am repeating:
find the upload button, create webelement of this and follow this code:
//open upload window
upload.click();
//put path to your image in a clipboard
StringSelection ss = new StringSelection(<give file location>);
Toolkit.getDefaultToolkit().getSystemClipboard().setContents(ss, null);
//imitate mouse events like ENTER, CTRL+C, CTRL+V
Robot robot = new Robot();
robot.keyPress(KeyEvent.VK_ENTER);
robot.keyRelease(KeyEvent.VK_ENTER);
robot.keyPress(KeyEvent.VK_CONTROL);
robot.keyPress(KeyEvent.VK_V);
robot.keyRelease(KeyEvent.VK_V);
robot.keyRelease(KeyEvent.VK_CONTROL);
robot.keyPress(KeyEvent.VK_ENTER);
robot.keyRelease(KeyEvent.VK_ENTER);