From: Analyzing the symbols in the cave

I tried lightening and increasing the contrast on some frames to see if that helped clarify things.
It doesn't, at least not much.

I love symbols, and From has a bevy. Not only do Victor and Miranda paint up a storm of pictures, the evil creatures paint their narratives in their caves. At least, Victor assumes it was the monsters who painted them. 

Some of the pictograms are easy to decipher, others... not so much.

This is my take on what they might depict.

Let's take it as a given that the tableau goes from left to right. 

The far left side of the mural, showing the river. Has this been supplanted by the the road?

At far left, we see a river with three boats carrying 2-3 people each (8 in total). There is forest on either side of the river, and 3 large conical objects that might be stones, all in white. They may reference the red stone circle (prior to it becoming red). Below this is a large red structure that looks a little like an Inukshuk, but has rays going from either side of the top: this is the lighthouse. 

At the right end of the river is a massive tree, taller than any of the others, and in black, rather than white; possibly the tree the children poured their hopes into, for all the good it did them. Above are two red curves cupping an empty centre, matched at bottom by two similar curves, but this pair is ringed by trees. Possibly cave entrances/exits. Could be Faraway Tree exits, too, but then, why would one have trees around it, but not have a tree motif incorporated around the curves?

The boats (canoes?) are the only mode of locomotion depicted in the entire tableau, which suggests this was the original way to enter the Hell Bubble. The river has since been replaced by the road we know so well.

At the end of season 3, it groes cold and snows, after decades of warm, stable weather. That there is a landlocked lighthouse, and no longer any river (or sea) nearby, suggests that the changing of the weather might extend to the geography itself. The buildings may be replaced, and new buildings may appear. 

We already know this is a curated space (Tabitha was let go to snag Victor's dad and bring him back, Boyd was brought in to organize the survivors, the priest to give them hope, animals appeared out of the forest to provide sustenance, etc), and the geography has changed before, so why not again?

I don't think the monsters painted the mural. This was painted long before the 1950s, likely back in the days of fur traders and Courier de bois. The drawings look more like tribal pictograms than what an educated person from the 20th century would scribble. 

That raises the question: are the monsters the originals, or new iterations? If they were created centuries ago, is their Fifties garb an affectation? Do they 'reskin' themselves to unsettle more modern inhabitants of the town? 

The circle with three roots, the iconic symbol Jade sees in his dreams, and that Miranda paints, is next. Beneath it are 7 white rectangular sacrificial slabs, one for each of the seven children. Below the slabs are 2 figures slightly to the left, and 8 figures to the right. Note there are also 2 more figures in the upper left, just above the circle symbol, which are easy to miss. They may represent the archetypes Miranda and Christopher, in their 'original; past life, fleeing from the sacrifice. They may also be the two below who are to the left, perhaps suggesting they were trying to undermine the sacrifice. 

There are 8 white figures in the canoes, and 10 figures beneath the circle (albeit 2 are slightly separated from the 8). The number of figures varies across the tableau, so it's hard to say if this has any significance. 

Next we have, at bottom, a tree, out of which fly four crows or ravens; beside it, towering over everything, is the Big Bad: a crude, menacing shape that seems to have 4 legs and two stubby arms. It's blood red and outlined in black. This is the demonic being to which the children were sacrificed. Did it also create the pocket universe? Or was it trapped here by another power? Not sure. But the pocket universe could be akin to a spider's web, designed to trap prey, so the Big Bad can feed off their fear and pain. 

To the upper right of the Big Bad is a black square, out of which fly 11 white birds. Not sure if the colour use here is significant or not. 11 does not match the number of any of the previous figure sets. These are likely the crows that appear at the fallen tree on the entrance road.

Below the box are two figures, holding hands. They are off on their own here. Likely MIranda and Christopher ancestors, continuing their flight. 

To the right of the Big Bad: crops, red figures, and infinity

To the right of them is a forest at top, with a white house, and beneath it rows of crops, surrounded by red figures, some of which appear to hold spears. These could be aboriginals, or they could be red because they are the monsters. There are 14 figures in total. Why are they surrounding the crops? Are they farming, or are they raiding? Not sure. The white house could be one of the abandoned houses by the second food source, the one with the freaky warding heads on sticks. But there are 3 houses there, not one. So I am not sure. 

I think the mural was painted long before the modern town was plunked down, like decorations in a fishtank. This seems to represent the original tragedy, which occurred hundreds of years ago. The big problem with this reading is the garb of the monsters: if they are immortal, you'd think they'd be wearing pioneer garb, but they aren't. If the Big Bad grants immortality to people who sacrifice to him, what happened to the earlier generations of monsters, the ones from centuries ago? Did they make a different compact? And yet, it is explicitly stated that the children were sacrificed so their parents could live forever, so the monsters are the original sinners. Their costumes then make no sense, unless they are projections or something. 

In the far upper right are two figures painted in white, holding hands. Again, Miranda and Christopher. Beneath them is an infinity sign with a sidweays line through it surrounded by 'glow' lines. I suspect this represents Miranda and Christopher being tied together as the two sides of infinity, jointly recycling through this ancient trauma/crime. 

The lighting is honestly terrible, and my fidgeting in my phone app hasn't helped much. It's hard to make out detail, and I may have missed things. 

Finally we have the talisman stones (below). It depicts two people overlapping, with what could be a large belly, possibly pregnant. On the left side is dawn, on the right side, sunset. Around them is a ring of trees. The figures protect people in houses/structures from attack, so it could be the spirit of life/birth, or it may just be the merging of Miranda's and Christopher's spirits. Note the monsters twisted pregnancy into birthing Smiley, so it could be a larger thematic element.