I have been increasingly concerned over what I found at the back of my throat today which I took a picture of (link below)

http://imageshack.us/photo/my-images/692/photouao.jpg/

I have no idea what it is and it's not causing me any pain. I have had occasional canker sores and have only recently that I was examining and saw this. Is it normal? For it to be like yellowish at the back? Something I ate? Or is this something I have to worry about?