All your media can be found in the Media Library within the Content area. To go to your media library, click on Media in the sidemenu on the left.
Here you can search through your videos, audio files, images and avatars. When you click on a media item you will see where it is used within your content. If the media is not used, you can delete it by clicking Remove.
Media (images, audio, video and avatars) can be uploaded within the activity manager while creating activities, but can also be uploaded from the Media Library by clicking Upload and choosing the type of media you want to upload. In Content Blocks you can learn more about uploading media files.