I think this is right but i am not 100% sure!! lol
Click on "Go Advanced", down there ↓↓↓↓↓↓↓
Then-On the top row (under "Message"), a couple of icons along from the one where you change your text colour is a picture of a paper-clip (When you hover over it, it says "attachments").. Click on that and it gives you the option of uploading from a URL or from a file from your computer... After that you are on your own!!! pmsl x x
I hope that helps.