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PHP Code:

function onCreated() {
  this.db = findnpc("db_registeredguilds");
}

function onActionServerSide(action, acton) {
  if (temp.action == "getImage"){
    triggerclient("weapon", this.name, "getFlag", this.db.guilds.(@player.guild).flag);
  }
}
//#CLIENTSIDE
function onActionClientSide(cmd, data, data2) {
  if (cmd == "getFlag"){
    Stan_Window_Image.image = temp.data;  
    this.guildflag = temp.data;
    player.chat = temp.data;
  }
}
function onCreated(){
  triggerserver("weapon", this.name, "getImage");
}
new GuiWindowCtrl("Stan_Window") {
    profile = "GuiBlueWindowProfile";
    useOwnProfile = true; 
    extent = "495 300";
    x = 50;
    y = 50;
    text = "Guild Registration Lobby";
    visible = true;
    canResize = false;
    new GuiShowImgCtrl("Stan_Window_Image") { 
      x = 12; 
      y = 50; 
      width = 150; 
      height = 150; 
      image = ""; 
    } 
} 


No need for the DBNPC code using this.
Also make sure object names match; you had a "_" at the end of "Stan_Window_Image" in the clientside trigger.