Another option, similar to Popplet that can work as an online visual bulletin board for students to gather inspiration from. Pintrest might work better as they can add to their own board and also search within other people's pin boards to pull ideas from.
Students can use these to collaborate within and between groups on their ideas for the mural. They can also add links to various murals they find inspiring and write about why that is. They can also work together using Google Docs to create a presentation of their idea for school administrators when they are ready for that step.