Travelling inside paintings?

I read this book a few years ago where people could go inside paintings.

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It was set in medieval times, and the main character was this kid from a small village. He wanted to go to the painters' guild in the capital city (?) because colours were really expensive in the universe it was set in. SO he joined the guild, and then he got painting. Then he found the office of some professor or someone, and there was a painting with a symbol on the back of it. Then he drew that symbol on the back of one of his paintings (a black and white, poorly drawn landscape), and he went inside it. He was being chased by some big monsters through grass tunnels, and I think he had a female friend. Later on in the novel, there were a bunch of people trying to get inside this painting with a massive green pyramid in it (I think that was also the front cover of the book), and there was treasure in the pyramid, or something like that.

Thank you for reading, and I appreciate any help.