Yes it looks like it only accepts from panoramio. Google photos need to be reviewed by Google and meet its policies before they are accepted. And it takes about a month or so for it to appear even then. So I think it is unlikely you can have an app on your site that lets you upload to Google Maps. Panoramio has a web api but it doesn't allow uploading, only downloading.
You can find how to upload here - http://www.squidoo.com/uploadGoogleEarth Google's policy is here - http://www.google.com/help/photospolicy_maps.html Panoramio offers free accounts and about 2gig of photos you can create an account here -
http://www.panoramio.com/ Panoramio API is here - http://www.panoramio.com/api/widget/api.html