Prevent Google from uploading pictures when connected to wifi? I noticed that pictures I took with my Android smartphone while connected to my home ADSL/wifi router were silently uploaded to Google Photos.

How can I disable this "feature" ?

Thank you.
To add to the answer Alex has posted (which is correct), if you still want to upload a particular photo to Drive after disabling sync, long press the photo- (select it)  and you will see a share option using which you can share to your drive.

This comes in handy when you have globally disabled sync for photos but want to save some photos to drive