Three ways.
Posting an image already hosted: Right click the image, copy the URL, then tag it with the IMG button.
Uploading an image: Go down to the line right above the "post now" button, click "Browse" and find whichever image or file you want to upload onto the website. You can only do one of these per post.
Signature: Like posting an image already hosted, just go to My Controls under Signature Settings.