You can change it like any other variable of an object
PHP Code:
MyImage.image = "block.png";
Stan_Window_Image_0.image = "block.png";
if it's a dynamic name, you can do
PHP Code:
(@ "Stan_Window_Image" @ this.guildFlag).image = "block.png";
However, you shouldn't be placing the image name in the GUI control's name. Instead, try appending numbers (_0, _1, etc)