It looks like all photos you can see on the default googlemaps come from panoramio. Is this the only provider? Can I create a website for uploading photos and show them on google maps?
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