I've been wrestling with the plugin not using the image I provide it and I think I've found out why. Bascially my theme, I've turned off...
...because I don't want to use post thumbnails on my blog posts. This however conflicted with your plugin which saves the image as the post thumbnail.
I'd advise that you change this so that you can just save any old image against the Advert, rather than use post thumbnails which may or may not be enabled by the theme.
Cheers for a great plugin all the same,