To add documents and media to a Card press the attach button 
in the top-right corner. From here you can either attach existing media from your Project, or upload a new file from your computer.
You can choose documents, spreadsheets, presentations, images, and videos.
You can find your attached media on the Card Media tab.
Tip: All images and documents can live on multiple Cards at the same time. If you update them, they update everywhere! So there is no need to duplicate and re-upload them. You can edit these documents at the same time as another team member and it will update for them in real time.
Note, the first image that you attach to a Card will automatically become the main Card image.
You can always select or upload a different main Card image by clicking on the camera icon. For more information on the main Card image, click here.