To upload media in mobile editor
On the scene, select the switch 
icon to start uploading or selecting your previously uploaded media. Then select "Media" to see the media options.
The select the upload icon 
to upload your own media. If you have previously uploaded media, you can filter the Media to "Uploaded" to see only the media you have uploaded.
To upload media on desktop editor
On desktop you can upload your own image to a scene by clicking the 'Uploaded media' tab on the list side and then clicking the plus button:
