I created the Admin folder within the project and created other pages within it (for example Default.aspx). The Admin folder has security set within web.config to only allow Admin. However, when Admin/Default.aspx is displayed in the browser all the images that are part of my .master page file are not displayed.

That doesn't make sense since the master page has a content placeholder in it for the content to display. Any thoughts? Seems like a simple issue?

I had a similar problem. I am guessing that you have used standard html img tags for your images. This is fine in the master page or pages at the same level of the master page, but as soon as you start adding pages in subfolders the url to the image in the new page is not correct. To fix this I used asp:Img control instead and it is able to resolve the url relative to the root folder as long as it is set to runat="server" and ImageUrl property has the prefix of "~/" followed by the image location eg "~/images/my_image.jpg". I am a newbie at all this, and I may have got this wrong, but that's what worked for me.

Cheers,
Brian.