To add a logo to your school's Classlist site, simply visit the 'Manage School' page from the School tab on the Admin side of Classlist and click on the camera icon in the circular logo space. This will bring up the photo uploader dialogue box, allowing you to add your own logo.
You can also add a cover photo for your school; this image space is larger, and is ideal for pictures of the school itself. Again, just click the camera icon in the top right of the image area. You can change these images at any time using the same process.