I don't think it was literal images/pictures, but the philosophical idea that an image represents (but is itself not) another [authentic] thing.
Edit: An example:
You make a post on Facebook simply about your $HOBBY. I hit the "Like" button. Do I actually, authentically like that Hobby, or you talking about it, or both? Am I just pressing it out of a social obligation? I instead could have reached out and called or texted and had a conversation about your $HOBBY. And you could have then judged to what degree my interest and "liking" of your $HOBBY was authentic, and how that reflects my interests, or how I am trying to branch out and am taking a genuine interest in you-the-person. But in lieu of a real conversation, I hit a "Like" button and let the image of an authentic feeling of "liked-ness" travel to you, instead of taking the effort of conveying the actual feeling of "I like you" to you.
I think "authenticity" with regards to the society of the spectacle is also a kind of image, but I know what you mean.
I have in mind the modern trendy value on authentic things. For example bespoke craft foods. The striving for authenticity in these things are itself a type of spectacle.
Generally I struggle with Debord but with the Society of the Spectacle I think I've got it down to something like:
1) Where once Bob could be cool in himself and people wanted to be like Bob. Being.
2) Then people saw the cool clothes that Bob wore and got them. Having.
3) Now people just get the things that look like the clothes that the people who look like Bob have. Image.
For "authentic" foods, same thing. People made good quality foods. Then people got the same thing that these people had or bought their products. Then people bought things that looked like it was the same thing. Or people valued the look of having these things. Image.
I think it's basically consumerism. But I suspect it's something deeper and applies to non products also. For example the image of being a good person, or a useful employee.
With regards to social media, clicking "Like" could be an authentic emotional response to an image. "I want to be like this"?
I think that people are fundamentally trying to find ways to love and approve themselves. They think that's found in Being. (It's not - Bob may not have it, either, but people think he does.) But they don't have that, so they try to buy it with Having or fake it with Image. And it works, a little bit... but not really.
Edit: An example: You make a post on Facebook simply about your $HOBBY. I hit the "Like" button. Do I actually, authentically like that Hobby, or you talking about it, or both? Am I just pressing it out of a social obligation? I instead could have reached out and called or texted and had a conversation about your $HOBBY. And you could have then judged to what degree my interest and "liking" of your $HOBBY was authentic, and how that reflects my interests, or how I am trying to branch out and am taking a genuine interest in you-the-person. But in lieu of a real conversation, I hit a "Like" button and let the image of an authentic feeling of "liked-ness" travel to you, instead of taking the effort of conveying the actual feeling of "I like you" to you.
This is the core problem with social media.