This is a folder within the folder you are publishing from. To start out with, use the complete path, and parse off what you don't need after you understand where it is pulling it from. So in other words, instead of "img/mountains.jpeg", use "E:\webfolder\WWW\img\mountains.jpeg".
After you do that, make certain the SERVICE that is retreiving the image, has read rights to the folder! You may be dealing with a permissions issues. Do you get any errors?