Help the art detective complete her incident report about the missing masterpiece.

Take a close look at the woman in this famous portrait. She is clearly waiting for her mysterious lover, who is hiding in the background.

Take a close look at the woman in this famous portrait.

When talking about 2D artwork or photos, English speakers say someone or something is in the picture, not "on" it.

She is clearly waiting for her mysterious lover...

The correct verb phrase is "wait for someone." Using "wait on" actually means to serve someone food (like a waiter), and "wait after" is incorrect.

...who is hiding in the background.

"In the background" is a fixed phrase in English. Slavic speakers sometimes omit the article or use "a", but specific areas of a picture (the foreground, the background) require the definite article.