If you mean link to a picture from another site so that it shows up in your post, you need to right-click the original picture and go to properties. Copy the pic's URL and encase it in the IMG tags like this:
If you mean how do you put a picture from your own computer in, you have to do the same as above but only after getting your picture online. www.imageshack.us is incredibly simple to use. Once you've uploaded the pic, link to it as I explained above.