If you’ve tried the Auto Backup tool and you want to delete your photos, it won’t be easy. You can go to the Auto Backup section of Google+ Photos, mouse over a photo, click the “tick” icon, select photos one by one and click “Delete”. You can also select photos by dragging a “lasso”, just like in your favorite file manager. You can also select a photo and Shift+click the last photo you want to select or Ctrl-click photos for non-contiguous selection.