Salam Alaikum
As such there are no symbols used in Islam to represent Islam itself - however the cresent and moon seems to have been adopted to represent Islam - though it has no significance in light of Islamic teachings.
However banners were used in the time of the Prophet(sa)
- Green banner (plain)
- White banner (plain)
- Black banner (plain and also with the Tawheed written on it)
Is it for any specific purpose? Such as an organisation or group you're starting. In which case we could help with ideas. Historically though, as brother Akulion has said, plain banners.