I have to create an essay using a symbol from The Metamorphosis. Do you have any suggestions?

My first suggestion is to pick one symbol. That might sound silly, but if the prompt specifically said "a symbol," don't write an essay that talks about three symbols. The prompt is forcing you to really analyze a single symbol. This forces your hand a little bit. Don't pick a small, minor symbol. There simply won't be enough textual evidence to explore and support your thesis statement. I definitely recommend having a thesis statement that makes an argument about the chosen symbol.
Perhaps make an argument that the picture of the wealthy woman in furs is symbolic of Gregor's hopes and dreams and how that symbol remains despite the fact that Gregor is now a bug. Readers are introduced to the picture in the second paragraph of the story. Your essay could discuss why Gregor had the picture there at all in his human life, and then the essay could discuss how the picture becomes even more important to Gregor in his new bug life (perhaps because it represents beauty and a time when he was human).