Overwriting image on SSD1306 OLED without clearing display

I have defined two images in code. One is shown when wifi is connected/associated, the other is shown when wifi disconnects.

var imgNetworkSignal = {
  width : 8, height : 8, bpp : 1,
  transparent : 0,
  buffer : new Uint8Array([
    0b00000000,
    0b00000011,
    0b00000011,
    0b00011011,
    0b00011011,
    0b00011011,
    0b11011011,
    0b11011011,
  ]).buffer
};

var imgNoNetworkSignal = {
  width : 8, height : 8, bpp : 1,
  transparent : 0,
  buffer : new Uint8Array([
    0b00000000,
    0b00000000,
    0b00000000,
    0b00000000,
    0b00000000,
    0b00000000,
    0b11111111,
    0b11111111,
  ]).buffer
};

The wifi connected image is correctly shown as soon as soon as the NodeMCU I am using connects to Wifi. When I disconnect from wifi I expect the other image to show, but I still see the wifi-connected image. I need to specifically use a clear-command to remove everything (there is only a wifi-signal image) before showing the wifi-disconnected image. Strangely enough, it works the other way around. This makes me think that the zero's in the wifi-disconnected icon are not correctly resetting the one's from the wifi-connected icon.

var wifiDisconnectImage = function() {
  console.log("Disconnect icon should show");
  graph.clear();   // <- required to clear OLED
  graph.drawImage(imgNoNetworkSignal, graph.getWidth()-8, 0);
  graph.flip();
};

var wifiConnectImage = function() {
  console.log("Connect icon should show");
  // graph.clear(); <--- works without the clear
  graph.drawImage(imgNetworkSignal, graph.getWidth()-8, 0);
  graph.flip();
};

I think it's because you've set transparent:0 - so you're telling Espruino that anything that is 0 should be treated as transparent. If you just don't set transparent then you should be ok.

Good catch @Gordon, that solved it!