Images are a typical example of non-text portions in a web page. Probably 99% of the web pages contain at least one image.
Even from the days of WML it was clear that on a mobile device it was important to define an alternative text to images and objects in general. For this reason, since WML 1.0 the attribute alt is required for img tags. This is very important because on mobile networks it is not uncommon that users disable the download of images or that the browser does not support the object that the author originally embedded in the page. We always want to make sure that our mobile users have a good experience even if they have some limitation or chose to avoid downloading objects that might slow down their browsing or cost them more money. If the text alternative is clear, the user might decide to reload the page or download the embedded object manually, if they are supported.
We may not really call this a "technique" as it is simply the application of the basic requirements of xHTML. Defining an img tag and setting the alt attribute empty is just a waste of time. We have the attribute, there's a good reason to use, so let's use it!
This simple rule applies to every object you want to embed in your pages. Always remember that browsers and devices that will visit your site will be very different in capabilities and installed plug-ins. Most of them might not be able or might not want to install extensions, so text alternatives to non-text objects are very important.
Sample
<img src="http://example.com/images/logo.gif" alt="W3C Logo" width="100" height="50" />