This picture is from the 50 trillion dollars challenge from the free ebook "Muko and the Secret". This one is already solved by a reader.
My final answer is
I noticed that there are
smaller and bigger stones in the archway. Reading them clockwise I get
Then I tried to decipher that
as morse code. I found no hint how to split that up in letters, so I tried until I got a meaningful word:
–• •– – ••– •–• •– •–•• •••= naturals
I then noticed there is still more:
On the floor are also two lines that look like morse code:
I finally managed to decipher that.
It splits up as
–•–• •–•• •– ••• •••which reads as class