How to embed a static image resource in a web server control?

Question

I am going to create a web server control representing a treeview. So I want to use 2 images for + and - for expand/collapse. How can I build this into the control in a way that can be used as image source when rendered on the page?

Since this will be in a compiled web controls library, I don't want to rely on external images in the web application.

Edit:
Based on this answer by Andre Kraemer I did the following:

In AssemblyInfo.vb:

<Assembly: System.Web.UI.WebResource("MyCompany.MyWebControls.Resources.plus.gif", "image/gif")> 
<Assembly: System.Web.UI.WebResource("MyCompany.MyWebControls.Resources.minus.gif", "image/gif")> 

In my RenderContents override:

Dim lPlusImage As New WebControls.Image()
Dim lMinusImage As New WebControls.Image()
lPlusImage.ImageUrl = Page.ClientScript.GetWebResourceUrl(Me.GetType(), "MyCompany.MyWebControls.Resources.plus.gif")
lMinusImage.ImageUrl = Page.ClientScript.GetWebResourceUrl(Me.GetType(), "MyCompany.MyWebControls.Resources.minus.gif")
lPlusImage.RenderControl(output)
lMinusImage.RenderControl(output)

My Assembly name is MyWebControls.

My Root Namespace is MyCompany.MyWebControls.

The images plus.gif and minus.gif are located in a folder named Resources, and the images have Build Action set to Embedded Resource.

It still does not work. I get no errors. I have tried the generated image url directly in the browser, bot nothing happens, just a blank page.

Note:
I tried to use an invalid path in the resource name, and the result was exactly the same, which made me wonder if I need to do something special to map the actual resource to the resource name. I got a 404 Not Found error only if I used different name in the code than what was specified in AssemblyInfo, it had nothing to do with path was pointing to an actual resource!

Edit:

I found the solution!

I found out that it is a difference between C# and VB. See my own answer to this question.

Answer 1

add the two images to a subfolder called images of your treeview control project. Then change their build action in the property grid from content to embedded resource.

In addition to that, you have to register those two images as embedded resources for your assembly in the assemblyinfo.cs file like this:

[assembly: System.Web.UI.WebResource("YourProjectsNameSpace.Images.plus.gif", "img/gif")]
[assembly: System.Web.UI.WebResource("YourProjectsNameSpace.Images.minus.gif", "img/gif")]

As those images will be embedded into your controls assembly now, you can access them using the ClientScriptManager like this:

string plusImageUrl  = Page.ClientScript.GetWebResourceUrl(this.GetType(), "YourProjectsNameSpace.Images.plus.gif");
string minusImageUrl  = Page.ClientScript.GetWebResourceUrl(this.GetType(), "YourProjectsNameSpace.Images.minus.gif");