It seems that the problem is coming from your IIS configuration which limits anonymous from seeing content on the pages. Please check if you have set Allow anonymous access to this Web site
( check the attached screenshot ).
There is one more option that you can check if the images are not served, but its mainly concerning IIS. -uncheck 'Verify if file exists' checkbox located in your Wildcard Application Map setting.
Also please have a look at the following blog post
which might be helpful.
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