[flexcoders] determine if image source exists

[bhaq1972]

How can I check if a the jpg source defined by a string really exists. 
eg

var image1:Image = new Image();
var sHello:String = "images/hello.jpg";

if(sHello exists) // this is the bit i need help on
{
  image1.source = sHello;
}


tia
bod