Okay, copy and paste the link. If you don't know, to copy you select the link to the picture and then hit control + C. To paste, hit control + P. The link should look something like:
therx.com/jake.jpg
If it doesn't end in .jpg or .gif it might not post right. Some websites are tricky. Anyway, assuming it ends in one of those...
<img src="">
that is the code you need and it has to be exactly how I listed it.
Just past the link to the picture in between the two " marks and then post it.
That's an HTML code. There may be an easier way but this way has never failed me.