You click the upload file button and select the image file. You can do it from your computer or phone. I know you can upload files from Android, but I'm not sure about iOS (probably can). If you're on Android and want to upload a picture from your camera roll, they are in my files>device storage>DCIM>camera
Hope this helps
Edit: this has already been answered, but this is how you can do it from your phone if you didn't already know that