On the page where you create your topic and body of the advert you're posting, below that is your email addy twice box, and just below that is an "ADD IMAGES" clickable button. When you click it, it then opens up four image placeholders and allows you to add images.
Just go to edit your ad and look down towards the bottom, above the continue button. Should see the "Add Images" button there.
Example here.
Look just above the Continue button at the very bottom of the page.......you'll see the Add/Edit Images button.