Hi Christina,
I'm afraid that's not possible. The system was originally designed to disallow this, as we didn't want people to "game" the system with non-live images. This may change?
I have to say though I think the image you already have and the audio is wonderful! Please feel free to checkin again at the same location with just your new photo (or more audio!). What we do plan to introduce is grouping checkins together, sometime in the future :)
Cheers,
Geoff