When I was 10 or 12, Dungeons and Dragons was a big thing but the content was fairly limited. Not just the sheer number of modules, but the tone and such were limited by the player's personal preferences. Out of dozens choices only a handful leave an impression. Face it, it's really a chocolate, vanilla or strawberry choice. We like what we like for no other reason.
This creates a cycle were the player was introduced to the module, then they presented the module as a DM to replay. The story gains additional replayability through this introducing it to others. I can repeat by placing a twist on the source material so that it is disguised. By the time you have your own kids, you see the cycle start again. It is very much like some beloved concept such as drawing, woodworking, camping, Disney, etc. to be passed down.
The activity is the same, but different depending on where you are in the cycle. This naturally leads to the idea of maps, guides, handbooks, t-shirts. I find it amusing that there could or would be some sort of insignia, brochure or mission patch for an old module because the are simply loved and repeated.
That is the source of inspiration for these images and my love of the OSR.
The difference between 6 and 10 is not very great in terms of time, so I recall my bedroom decorated with classic Disney posters, 60's and 70's baseball pennants and other object de art which were done in these odd colors and styles. It's no wonder that I have associate these images with those conjured by D&D.