You can upload a profile picture in one of two ways.
1) Self-edit Email
If the event organizer has sent you a self-edit link via email, you can use that to access your profile on your desktop and add your photo there.
On the app, you can edit your profile and add your picture by navigating to the Login pane, logging in with the email you used to register for the event, and clicking the Profile button in the top-right corner.
If you are having any trouble uploading your image, please send the image to support and indicate your name and the event you are attending. We would be happy to upload it for you.