The reason images placed in the course don't show up on the main page (course listings page) has to do with privacy and security. To elaborate, Moodle does not allow outside access to course folders/files- just think if a private image or file was accessible to the outside world or to students not enrolled in a course.
The work around to this is posting files you want to be accessible for the whole site in the site files area. A file placed here can be accessed from within a course.
To access the site level folder login > from the main page select front page from the site administration menu > Site files > this area has controls to create folders and upload files.
The site folder is located in the moodledata directory and is folder 1.
Other possible solutions
- A workaround would be hosting the images in a separate location or site such as Flickr; however, this may not be as easy as posting the images to the site level folder.
- Another option would be using FTP to upload images to the Moodle pix directory; again, this may not be easier.