Why can't I delete old photos from my photo stream that were on my MBP (that I wiped and no longer own)?

There are photos still around on my photostream from my macbook pro. I sold (and wiped clean) the computer months ago. The option to delete the photos is greyed out. It's not a huge issue, but I have OCD about keeping my photostream cleaned up and there are a multitude of desktop wallpapers and other random things that I can't seem to get rid of. Any ideas? I unregistered my macbook pro when I sold it. Tried logging into icloud.com, iTunes, etc. and there was no photo options in sight.