nobeeul,
You'll basically need to combine the replies of both ZR and pesocaster.
The picture needs to be hosted on a website, you cannot link directly to your computer. Once there, copy the address of the picture. When you create a post just above where you type in thare are some buttons. Click the IMG one. Paste in teh picture address and then click OK. this will place a line in your post that basically resembles the address of your picture. Click on Preview post to make sure it looks like you want. If it does send the post.
Hope this helps.