There are no hard-and-fast rules. Let your inspiration and a sense of offering be your guides. You might, for example, include a statue or image of a Buddha, a bodhisattva, or Buddhist deity that inspires you and connects you with awakened mind. People often put photographs of dharma teachers and other inspiring figures on their shrines, or pictures of lost loved ones. Most Buddhist shrines also contain offerings to the buddhas and other realized beings. These are usually symbolic offerings of the sense pleasures, such as a candle (sight), incense (smell), a piece of fruit or cookie (taste), etc. Whatever it is, if it helps your practice-space feel more like “home” and inspires your meditation, it’s fine to include it on your shrine.