I want to post a commission I got from PunkPega that he only privately posted on patreon, but gave me permission to share publicly: https://imgur.com/1r6s1Qi Can I upload it as long as I make that fact clear with a link to the Discord message?

Generally speaking, if it's your commission, you can post it.
Keep in mind that in almost all cases, the artist retains ownership over the actual work, so they can still take it down.

With other paysite content, you would need to have an explicit permission to post that artwork to e621.
Not just "share it around", it does have to be specific to this site.

This works the other way, too. Some artists require commissioners to let anyone upload it. Others will do whatever commissioner asks, but charge more for private work.

https://i.imgur.com/1r6s1Qi.jpg
Question: Will you post it publically or is it too hardcore?
PunkPega 02/18/2034 8:12PM
I probably won't. I'll put it up on my Patreon, since pretty much everything goes there, but I'll keep it at that. You have permission to share it around however

Fixed the ImgUr link.

Yeah, wording implies you need to ask more specifically for posting on Boorus. Sounds like implying only stuff like person-to-person?

The artist granted me permission and posted the pic publicly. https://i.imgur.com/W7AHABT.png https://twitter.com/PunkPegaAD/status/1698864328437321825?s=20 The characters' owner is also okay with it. https://i.imgur.com/aie82eV.png

miam19 said:
The artist granted me permission and posted the pic publicly. https://i.imgur.com/W7AHABT.png https://twitter.com/PunkPegaAD/status/1698864328437321825?s=20 The characters' owner is also okay with it. https://i.imgur.com/aie82eV.png

Awesome!
I can't exactly verify screenshots or hidden tweets, but main thing is so YOU know for sure the artist won't show up pulling everything down.
You should post the post number here once you upload it.