If you are attaching an image from another website you click on this button in the quick reply box
and put the address in the pop up window.
If you are uploading an image from your computer you click on the post reply button
and scroll down and click on the manage attachments button and then click on browse to find your file. Click on the file and then click open. Then click upload and close the little window after you've uploaded the file. It is attached to your message now.
You can practice here if you want to.