You need to list what the message said to get effective help. Next time, please write it down on a piece of paper so you can tell us the exact words. Your uploads cannot have a large file size. c0wg0d said above: "Images can be up to 5 MB and you can insert up to 10 in one post." Maybe your picture was too large.
That means it's probably not a picture, or it's a picture we don't accept. The filename needs to end with .jpg, .png, or .gif.
Am I crazy? I am looking everywhere in the Minecraft Building Blocks emoticon symbols menu and I cannot find cobblestone. I find cobblestone stairs: and mossy cobble: but no cobble. Am I staring at it and just not seeing it? How can there be no cobble?!?!