Way of halting execution until certain event completes

[Mike Kypriotis]

I have a loop where Itest if each of a set of images exists and if so display it the function for it is

function checkImgs()
{
    if (document.images)
    {    
          var imgsrc = 'Lo.jpg';
          var img = new Image();
 
        img.onerror = function () {
          alert(this.width + ',' + this.height + " can't be loaded.");
          }
        img.onload = function () {
          alert(this.width + ',' + this.height + " is loaded.");
          }
 
        img.src = imgsrc;
    }
}
 

the plan is to set a variable inside the onload event and after calling checkImgs() to simply check that variable, problem is that program execution will not halt so almost always the script will proceed to checking the variable before the onload event fires and modifies it. And I cannot put my entire code to the onload event.Thought about putting the variable checking into a while where it won't exit while the variable is 0 (the events will either give it say a value of 1 or 2)but it is a good practice?

[Mike Kypriotis]

decided to put everything in those two events... another question is is it 100% sure that one of the two events will be fired, cause if not my script will come to an unexpected halt (omitting the onabort which is a user action and thus if the user wants to stop loading/execution of the page nothing will happen)