You know the famous
Now you can create your own! I've been using these as my avatar on twitter for example, but originally I started out by making them for my classrooms.
There are a few sites you can make these on. I use the The Keep Calm-o-Matic. It is so easy to use. You can change the colours of the foreground and background, adjust the size of the text and the poster and even change the image at the top by uploading your own.
You could have your students make their own, or just make some yourself to decorate your walls with.
Here are some of the ones I have made.