Hey ChangNoi,
You can post image, you're just going about it in the wrong way
You need to upload you picture to the internet first through a hosting website such as ImageShack, TinyPic or PhotoShack.
I'll use TinyPic as an example, so load up the website and click the "Choose File" button, find your file and then click upload (You'll need to get the capthca right too - proves you're not a robot) Then Copy/Paste the IMG Code.
If you're still stuck, give us a shout
