Depending on what framework you are running slimbox under, you can easily allow slimbox open an image as soon as the page has loaded. Simply add the following code (based on your framework)  into your pages’s <head> tags.

Mootools:
window.addEvent(“domready”, function() {
   Slimbox.open(“image-to-load.jpg”, “image caption”);
});

jQuery:
jQuery(function($) {
   $.slimbox(“image-to-load.jpg”, “image caption”);
});