Firstly the pic needs to be small, max 1024kb (1MB). If it's larger than this you can usually get your PC to save it with less detail; I use Windows Picture Manager, Picture => Compress => for Web pages, then Save As so you don't lose the original high definition picture.
Or you can use this :
USE THIS TO RESIZE http://www.webresizer.com/resizer/
Now to add your resized pic to a message. When you are editing the message, scroll down and you will see Attachments and other options. Click on this, then scroll down again and you will see Attach: _________________________ Browse. Click on Browse, find the file and click on it, now when you Post your message the picture will be attached.