Learn Spanish (Latin America): Él esconde el dinero detrás del cuadro.

When the word de comes right before the masculine word for "the", which is el, they always combine to form the contraction del. It is a strict rule in Spanish to never write de el together as two separate words.

Why does the first Él have an accent mark while the el in el dinero doesn't?

The accent mark is used to tell two identical-sounding words apart. Él with an accent means "he", while el without an accent is the masculine word for "the".

Where does the word "of" come from if the English translation is just "behind"?

In Spanish, the phrase for "behind" is detrás de (which works a bit like saying "behind of"). Because it connects to el cuadro, the de and el merge into del. You always need that de to connect detrás to the object it sits behind.

I learned that cuadro means "square". Why is it translated as "painting" here?

Cuadro does mean "square", but it is also the standard everyday word for a framed picture or a painting. Since you cannot physically hide money behind a geometric shape, context tells us it means "painting" in this sentence.