Does somebody know where I can find a good API to send and receive photos from my Android-phone to Picasa? I've been searching for more than 2 days but I can't find a good way.
I think the best way was to do this via HttpRequests and using OAuth2.
Does somebody have a good example on this?
They have a public facing API described here:
https://developers.google.com/picasa-web/docs/2.0/developers_guide_java
You are right. You will have to authenticate your app using OAuth and then send HTTP requests using the provided API's.
It should work on Android. If you need a sample on how to use restful web services on Android, You can see this sample from google:
It's a sample app that uses the flickr API to stream photos.