Rory (and others)
To acheive the desired effect make sure that the picture is on your PC (not on the internet).
Then...
When creating a post, click on 'Manage Attachments' in the Additional Options below the edit box
(SEE PIC 1)
Using the section highlighted, browse to the file on your PC paying attention to the file size limits detailed in the table below the input field
(SEE PIC 2)
Once identified, click the upload button
(SEE PIC 3)
Click close this window and continue posting your message
(SEE PIC 4)
Hope this helps, please reply if you need clarification 