Web Resources are a very nice features of .net giving the developer the capability to bundle a bunch of files into a single DLL for easy transport. However, it is not very forgiving when it comes to coding errors. There are a lot of sites out there that offer details on how to add a web reference, what is missed is just how precise each piece must be written. Unfortunately when it breaks the errors are simple 404's which are not helpful in the least.
Step 1: Include the file in your project, right click, to go properties, and set it as an Embedded Resource.
Step 2: add: [assembly: WebResource("DefaultNamespace.Folder1.Folder2.FileName.png", "image/png")]
This step has a couple of gotcha's. First the contentType (the second parameter) must be correct, WebResource.axd will serve up the file with this content type. So a content type of text/html used with an image file will result in a screen full of ascii.
Secondly the first parameter is the name of the file after it has been compiled. A reflector tool comes in very handy here for verification purposes, I use dotPeek. When compiled the file is named using the folder structure it is contained in prefixed by the Default namespace with periods as separators. The Default namespace can be found by right clicking on the project name and opening properties. Note: it is NOT the assembly name. Failure to perform this part correctly will simply result in the file never being found, again a 404 error.
Step 3: Use something like this:
Those lines allow you to get the URL for an image, or add a script reference at the beginning of the HTML output for the page. Notice how the filename referenced is IDENTICAL to the filename used in the [assembly: tag up above? If these two do not match and are not correct then it will result in a 404 error; although a URL or script reference may still be generated, it will not be the correct one.
Also, the GetType() call is very important as well. The type referenced must at the least be in the same project as the embedded resource, one person said it needed to be in the same folder as well, but I am fairly certain that is not the case. To play it safe I just use this.GetType() in the local file or Resource.cs file that generates the WebResource URL. I say playing it safe because I have been bitten before by renaming my Resource.cs file and having my typeof(PageName) reference a completely different, but still valid object. This resulted in no compile errors and a good looking URL, but always 404 errors.
It can be very helpful to determine which /WebResource.axd is the one you are looking for in your HTML source code. To help with this someone over at Telerik has created a nice page that allows you to decrypt the URL and see a more friendly name.
Something to look for in the friendly name is your assembly name, if it does not reference the assembly that houses the embedded resource then you probably screwed up the Type when generating the URL.