Perhaps I should put a sitemap.xml file in there or robot.txt, or put an index.html with listed links to each image in the folder?
Please tell me :)
That’s right, you should either create a sitemap file, such as sitemap.xml, using the sitemap format, and submit it to Google, or simply create an HTML file with listed links to each image. Don’t forget to link to that HTML file from somewhere on your site, so that Google could find it out.