This small application works for me:
@namespace s "library://ns.adobe.com/flex/spark";
<s:Panel id="loginForm" title="Pylon Essentials">
Does that work for you?
That works for me too. It seems likely that your path is incorrect.
If you try embedding the Image files do you get a compiler error saying that it can't find the file?
You might also want to check your bin-debug folder to make sure the image files are being copied over.
You shouldn't have to embed the images, I just suggested doing that hoping the compiler would complain because I suspected that your path was incorrect, which apparently isn't the case.
This project seems to work for me. What build of the SDK are you using? You can quickly find the build number by putting this into your application:
<s:Label id="v" creationComplete="v.text = mx_internal::VERSION" />
My build version is 184.108.40.20685. I think that's the original SDK of Flash Builder Beta 2 when it is released publicly.