If the image is already on the internet, go to that page and right-clikc on it, then select "properties". It will show you the "URL", a string that begins with "http://" and ends with (usually) "jpg". Highlight that entire URL and copy to clipboard (Ctrl-c).
Hit "Post Reply" here and in the box type:
<img src=http://www.uvboston.org/images/sept_14_franda.jpg width=600>
I used the same image whose URL you posted in the body of the article.
If the width of the image is less than 600 pixels, you don't need "width=600". Simply post
<img src=http://www.brightoncatholic.com/images/StColumbkille3.jpg>
If you want it centered, denote the part of your post that is to be centered like this:
<center>Centered part
</center>
Once you start using HTML tags, you also need to use paragraph tags:
Para 1
<P>
Para 2
Without them all your text will appear as a single paragram, no matter how you format it in the "Your Reply:" box.
There was an HTML sandbox thread around here, but I cannot find it.
Welcome to FR.
The old "Bootcamp" thread doesn't seem to be around any more.