Auto Backup gone crazy
If the rubbish/random images aren’t in the camera roll on your phone, how are you determining they are coming from your phone?

I’m wondering if one of two things is going on: 


you’ve not signed out of google+ on some other device and that device is doing the uploading (sold or given away a phone or computer? used a machine or phone of someone else’s?) or 
you have a security app installed and the app has been activated and taking random images (and helpfully sending them up to google) to help you nab the “thief”.