What do the painted symbols mean?

There are a lot of "mural paintings" in the game, obviously drawn by the Solarii Brotherhood cult.

Some of those seem to tell a story about the sun queen or seem to indicate that the Solarii are there against their will (prison-like lines counting the days), but others seem to be indicators targeted at the player.

The first one seems to indicate that a hidden tomb is nearby, I have no clue what the second is supposed to say.

What helpful symbols are there in the game and what do they represent?

The first image is definitely a nearby tomb indicator, and is very indicative of the Solarii.

The second image definitely doesn't match the earlier one and is just before the sniper scene on the bridge (with Roth looking out for you with his rifle while you scoot along the edge of the bridge).

Might be foreshadowing, or just added for the "Crazy cult" vibe. Maybe they needed a target to practice with!

I believe the second may indicate that a challenge item is near by. I remember seeing this symbol right before I found one of the silencers. The silencers are the challenge item in this map. If I remember right, that symbol might've been the same symbol as the silencer from your map.