JQuery :: Next Step Executed Before Image Attributes Available

In the following code - Sometimes I get the correct image width and sometimes I get an image width that is ZERO. How do I make sure the loaded image attributes are there before I access them. Odd thing is if I click the same thumbnail twice I always get the width. If I click a thumbnail once I may get the width this time BUT when I refresh I may get a ZERO width.

Here is the code...
$(document).ready(function() {
$('#gallery a').click(function(evt) {
evt.preventDefault();
var oldImage = $('#photoBox img'); // Selects the current image
var newImage = new Image(); // Create a new image
var imagePath = $(this).attr('href'); // get the path to the image file to load that was clicked - via its href attr name
newImage.src = imagePath;// Set the newIamge.src to file name or Path
if (newImage.width < 600) // If the width is less than 600 calculate a new margin to center the new image {
var newMargin = (620 - newImage.width) / 2;
var newHtml = $('<img src="' + imagePath + '">');
$('#photoBox').css('margin-left',newMargin);
$('#photoBox').css('width',newImage.width);
} else
// assume width is 600 {
var newHtml = $('<img src="' + imagePath + '">');
$('#photoBox').css('width',newImage.width);
$('#photoBox').css('margin',5);
} newHtml.hide(); // hide the newHtml before prepending will fade it in
oldImage.hide(); // hide the old image
$('#photoBox').prepend(newHtml); // prepend the newHtml
newHtml.fadeIn(1000,function() { }); // now fade it in
});});