To share the photo from Flickr select the photo from your photostream so you see the photo on the screen. In the bottom right you have some icons. A group of 2 and a space and 3 icons together. The rightmost icon in the group of 3 looks like this.sharing
, on Flickr
Click on this icon. It opens up a little menu. Most likely it's showing you icons to share the photo on Facebook, Twitter, ...
From this menu click on "Grab the HTML/BBCode" at the bottom of the list. You get a new portion of the menu open. A text box at the top and below that you can select the size and if you like HTML or BBCode. Select the size you want (maximum size 1024) and select BBCode. It's difficult to see the names for the radio buttons so it's the right one.
Then copy the code you see in the text box. Paste it in your message here and the photo should appear in your message.
If you have problems with posting the photo please let us know so we can help you further.