Web
Click the 'Upload Media' button in the top right corner.
Select the team you want your media to end up in and add a description and some tags.
Drag or navigate to your files and hit 'Upload'.
Mobile
From the team you want to add photos or videos to, tap the '+' icon in the top right corner.
Select media from your phone gallery, add a description and tags (which will be attached to all selected media) and tap 'Import'.
Note: Images will be associated with their creation date, not the upload date.