First, save the photo to your computer and make a note of what folder you've saved it in (ex. My Documents\My Photos).  You may also need to re-size the photo - it must have a file size of under 2MB (that's 2000KB). 

As you're filling out the form to create a classified ad, you'll see a box labeled Photo, with a Browse button next to the box.  Click the browse button and, in the window that appears, navigate to the folder where your photo is stored.  Double-click that photo.  The window should close and now the Photo field will be filled in on the classified ads form.

Finally, click the Save button at the bottom of the form to post your ad and photo.  If you're attaching a photo to your ad, don't use the Preview button - it may not work properly with your photo.