Let me try my hand at answering that question based on what I do.
First I'm gonna "assume" you are using IE in some version. If some other browser this may not apply.
1. Put your cursor on the image on the other site and right click on it. Select PROPERTIES. Copy the URL of the image to your Notepad.
2. Put the URL you copied in the body of your message here.
3. Now place your cursor at either end of that URL in your post here. Click and hold the left mouse button and sweep across the URL to highlight it.
4. While it is hightlighted look up to the list of icons just above the message text box and find the one that has Img inside a box. Click on it. That will automatically place the proper code in the message box to make the image show up assuming the site you pulled it from allows remote posting of photos. Not all do.
GB