You just have to use the image tags around the link. Unfortunately if we try to type exactly what you need to do here, it thinks you are trying to post a picture, and won't let you. So I will try to explain.
In what you posted that started with IMG_3645, get rid of everything up to and including the quotation marks, and replace it with the letters img , surrounded by square brackets. Then, get rid of everything from the quotation marks on at the end of the link and replace it with /img , surrounded by square brackets.
Here's a picture of what your text should look like, showing the image tags:
I grabbed your link and modified it with the image tags, posted below. By the way, the information on posting pictures is also in the "FE Power Information and Rules - Please Read" sticky post, if you ever need to refer to it again.
