Right click on the picture on the internet, then type [IMG] the URL [/ IMG].

OR

When you go to make a post or a new topic, there is a little icon above the area that you type your message in that says "IMG."
Click that icon and paste in the link to the picture.
Press OK.

AND/OR

To host a pic from your computer, go to www.imageshack.us, upload your picture, then just copy and paste what they have typed there for you.