Adding images to ads is possible with Markdown formatting.
You need to put the image somewhere on the web, on a publicly accessible server. If you don't have your own place then you can use a free one: https://imgur.com/
Select option "New post":
On the next page, upload a picture, as instructed
Then right-click on the image and download its link. It is important to click on the image and download its direct address (the link should end with the extension eg. .jpg or .png).
You can use the downloaded address in the content of your ad using the magic syntax: 