https://support.aspdotnetstorefront.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=123

I think you should try it with "/images/"

Just to follow up on this, Cliff's problem was related to the Url Master module filtering out all .jpg images for rewriting purposes.  Because of the regex filter didn't have a 'end string terminator' on the end, it was detecting the .jpg in the middle of the request, and stopping the Url from being processed by the Rewriter.  While the Url itself doesn't need rewriting, my guess is that the ASPDOTNETSF component is picking up context information (probably portalsettings) and determining if the current user is authorised.  When the portal settings are not set (due to the request being bypassed) the error shows up.

Changing the Url Master 'ignoreRegex' filter to \.jpg$ fixes the problem - it has nothing to do with file permissions, the error message is a bit of a red herring.