Thank you for your question!
You can turn off the Place ad, Edit ad, Browse ad links by going to Classifieds > Settings > Ad/Listings. All the way at the bottom of that page, you can choose to have those items not show using the settings there.
That said, right now we don't have a way to replace those with an image in the plugin. However, you could easily add images to your page using regular HTML and linking the images to those functions.
I hope that helps. Please let me know if you have any other questions or issues!