To post an image you have to upload it onto a server you have access to. This means if you were to put that URL (address) in your browser (at the top) you would see the image on a blank page.
The address of the image between the tags in your post would look like this
http://www.mydomain.com/folder/nameofimage.jpg
Maybe what you need is:
A free web page editor, to get you familiar with protocol.
A free FTP program to upload stuff with.
A free amount of web space which will accept images.
There are lots of places you can go, one is www.tucows.com
Greetings Kanga