Asp.net Images随机提供401个未经授权的错误

I am using Asp.net, IIS 6 and cdn for serving images. I am having a problem with loading the images in my website. images are under a virtual folder. Everytime I hit refresh of the browser, some of the images gives 401 unauthorized error. (images and number of images displayed which gives 401 error differs in every refresh.) I am having hard time to solve this problem. Any help will be welcomed a lot.I am using Asp.net, IIS 6 and cdn for serving i