How do you diagnose a Access Denied Error?

On my company website images are not appearing inside a SSRS report deployed onto the website, and when I go to the images directly by right clicking where the image should load, this is what I get -

Access Denied
You don't have permission to access "http://MyCompanyWebsite/Reserved.ReportViewerWebControl.axd?" on this server.
Reference #18.84f7d217.2385168512.7ae64afb

The key piece of this that im looking at is Reference #18.84f7d217.2385168512.7ae64afb

I have searched the IIS logs(The one in IISLogs/W3SVC1) and event viewer(Application, Security, System) on the server that host the website. I can't find anything on where this would be logged. And if theres a Reference # then there must be something that has logged this.

Right? Or am I wrong in my train of thought? If anyone has experience in diagnosing this issue that would be fantastic. Thanks for reading.