Should the questioner continue to hang on her phone a medal with a drawing of a tree leaf for decoration, knowing that it is used in Western societies as a symbol of luck, and that her intention differs from that?

It is not permissible to hang anything with the intention of bringing good luck or warding off evil, such as hanging amulets that were hung to ward off calamities. As for hanging the aforementioned picture on the phone for decorative purposes, without any false beliefs, the basic ruling is that it is permissible, unless false beliefs regarding this picture are widespread in the questioner's country. In that case, it should not be hung to avoid resembling those who hold such beliefs.