I did a quick search under the Cassini forums for "images" and found the following, which might be of interest (I'm not using Cassini myself, but I am considering exploring it sometime in the next year).
One of the more relevant posts might be this one:
http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=396550
In it, they identify Forms Authentication combined with denying access to anonymous users as the culprit.
You may also want to read this posting on the Cassini forum:
http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=507080
In it, [Killman] writes:
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I believe the issue is that Infragistics uses virtual folders to access images and scripts.
For example the virtual folder /ig_common maps to C:\Inetpub\wwwroot\aspnet_client\infragistics\
Cassini doesn't support virtual folders, so that's why it won't find the files. There are 2 solutions:
1) Create a folder named ig_common in your root web. Then copy the files from the folder above.
2) Use CassiniEx (shameless plug) as it supports virtual folders.
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Alternatively, look at the posting
http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&PostID=451334, where Killman also writes:
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In Cassini, unlike IIS, all files except for the /aspnet_client scripts are passed to the HttpRuntime. In IIS, only those extensions that are mapped to the ASP.NET Isapi Extension will be processed by the runtime.
The problem with this is that if you enable forms authentication and static resources like images, stylesheets, and XML files are stored in a protected folder, ASP.NET will deny access to them until the user is authenticated.
I noticed the first time I enabled forms authentication and set <deny users="?" />. When I was sent to the Login page, none of the images or stylesheets were loaded!
To work around this:
1) Create a new folder called 'public' and move all your static files there
2) Add a new Web.config file in the 'public' folder with the contents:
<configuration>
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
</configuration>
...
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I hope some of this is useful :)