If you know the name (and path) of an asset (say, /logo-fb.png) and you want to access a known rendition directly, you don't need to use WebvantaScript. Instead, put the special path rendition.rendition_name in the path as a prefix:

http://www.webvanta.com/rendition.thumb/logo-fb.png

The rendition_name is the name of the desired rendition as defined in your site's settings. The example, the rendition name is "thumb".