Look for the Additional Options... just below the box you type your comments in. Click the plus sign. Press the Choose File button by the Attach:. This will open a box that allows you to find the picture on your computer. It must be saved to your computer before you can attach it. Once you have selected the file from your computer, it will show the file or file names after the Choose File button. Once you press post, it will be visible. It's that easy!
Please be aware that the site will not let you post pictures without something in the comments box. (Thank you to Foxx4beaver for catching that)