The EXTRA_ALLOW_MULTIPLE option is set on the intent through the Intent.putExtra() method:
intent.putExtra(Intent.EXTRA_ALLOW_MULTIPLE, true);
Your code above should look like this:
Intent intent = new Intent();
intent.setType("image/*");
intent.putExtra(Intent.EXTRA_ALLOW_MULTIPLE, true);
intent.setAction(Intent.ACTION_GET_CONTENT);
startActivityForResult(Intent.createChooser(intent,"Select Picture"), 1);
Note: the EXTRA_ALLOW_MULTIPLE
option is only available in Android API 18 and higher.