First you need to upload your pic to a web site. I use Photobucket but most any will do that stores the pic on its own server.
Then you need to copy the url of the pic and paste it into the body of the message you are going to post.
But unless you put the secret code before and after the url it will just show the address and not the pic.
Here is an example:
If you click 'quote' in the upper right you will see the format.
Good luck
SB