I managed to get a glimpse of said image by moving away all my songs: 
This is it. I have seen this image on the first time you click "Play -> Solo" during each session, or when you get into song select with no maps imported at all. It's different from the default, seasonal or skinned menu background we see every day, and it seems like this weird, rough and somehow creepy image is more like some sort of fallback or transitional image (placeholder) just as some workaround or something.
I'd love to hear more about the history of this image.
This is it. I have seen this image on the first time you click "Play -> Solo" during each session, or when you get into song select with no maps imported at all. It's different from the default, seasonal or skinned menu background we see every day, and it seems like this weird, rough and somehow creepy image is more like some sort of fallback or transitional image (placeholder) just as some workaround or something.
I'd love to hear more about the history of this image.