Thousands of years in the future, after a catastrophic pandemic a single population of humans survive. At the time of the event these people escaped because they lived in isolation in mountainous terrain and had no contact with technology or indeed with other humans.
Thousands of years later, these people still survive but are terrified of leaving their lands. This is because during the pandemic and for a long time afterwards, no-one who ventured out ever returned. There has built up a strong taboo about leaving and anyone who tries it is considered insane even if they do return. They are not allowed back past the sacred borders and no-one believes their tales about things they have discovered.
One day a young woman is digging for root vegetables when she discovers a mysterious artefact. She brings it back to the elders who decide they will open the thing. They recognise it as being some sort of box and it has a simple catch that is not locked.
The elders call a meeting of the local clan and, after suitable rituals have been performed by the medicine woman, they open the box.
One of the artefacts
Inside the time-capsule (for that is what it is) they discover many objects, some covered in writing they don't understand. Something that they do understand is pictures. Here is the mysterious picture.
What can a pre-industrial people make of this picture? Clearly it is of great significance. How can a primitive society comprehend or explain what they see?
I didn't want to advertise. However for those who wish to search for the original picture, this is an advert for a Ford Fiesta. I'm not sure how to give a link because it is a popup ad.
EDIT in response to a comment.
The very reason that these people survived the apocalypse at all is that they live in a remote mountainous area that is fertile unlike the surrounding area which is virtual desert for hundreds of miles.
Originally (before the apocalypse) people would set out across the desert to explore. They would either not return or they would come back with stories of wonder than no-one could really believe. (This actually happens with remote tribes). The people had no awareness of the world-wide plague. They simply discovered one day that no-one came back any more. Parents began to tell their children not to go. Eventually the elders prohibited leaving. Over the generations the people forgot why the rule was there and the prohibition became a taboo. The action takes place hundreds or even thousands of years after the plague and all the old stories are now myths. There are some items that explorers brought back in the old days. However there is no high-technology - it would be useless to them. Wanderers typically brought back things that were easy to carry and especially useful. These included clothing, knives and so on. Very little remains after all this time and the items are already revered as belonging to some unknown people - perhaps the ancestors.
Note - As a result of Michael Richardson's answer, I see that I should point out that the time capsule was originally a box of mementoes brought back by an adventuring tribesman who kept it to himself and buried it because he did not want to hand it over to the elders. He would occasionally go and dig it up to reminisce and then bury it again. He never revealed it to anyone else and died before he could pass on the secret to his children.