Behaviors/JQuery: What's the standard/convention in Drupal 7 for running Jquery code on event *OTHER* than DOM ready?
Using .load() on element to attach event handler in your behaviors should not be a problem. Using it directly to window outside a behavior, for instance to wait for all images to be loaded, should be okay too.

As pointed by Clive in the comment, Drupal behaviors are to be used in replacement of $.ready(), not $(window).load(). However, if content (ie. new DOM elements) is added to the page after page load (AJAX calls, content generated from JavaScript, etc.), handlers for $.ready() will not be called and will not have a chance to process the added DOM elements. While the attach() method of all behaviors is also called when this happen. With a parent of the added DOM elements as context.

Behaviors can also have a detach() method which is called when DOM element are removed or before form serialization. This us, for instance, used to properly destroy WYSIWYG editors when their textarea are removed from the DOM and to prepare the POSTed data hen their form are submitted.

Off course, this rely on the JavaScript doing the addition/removal/serialization to properly call Drupal.attachBehaviors() and Drupal.detachBehabiors(). Something that is done by Drupal core (including form handling and the Drupal AJAX Framework) and properly written contrib modules.

I'm not familiar with the load event and its behavior with content added after the first time it is triggered. If it does not fire multiple times, re-registering the handler in a behavior attach() method may be required to ensure newly added image are also processed.