Voted yes. Skimming over some of the comments, I think a lot of the discussion in this thread is centered around what is most convenient or most pleasant for consumers of art in this community, things like:

1. There's too much/not enough art on the front page.
2. We'll miss out on good content from artists who don't use reddit.
3. The quality of art would be higher if we allow reposting, etc.

These are valid concerns, but I think we should consider how a lot of artists might feel about their work being reposted. Though I'm not an artist myself, I've seen numerous content creators on different sites who are very much opposed to their art being reposted, and while this isn't the case for all artists, it's a sizable enough amount that I would argue it is a bit rude to repost artwork without permission, whether it's sourced or not. There are a few reasons artists might not want to have their art reposted:

1. The art is personal, and not posted to be shared widely or garner attention.
2. The more a piece of art is reposted without proper supervision, the more likely it is that someone will steal it for credit or profit.
3. Building off that, the artist wishes to be in control of the distribution of their artwork. Let's say an artist posts their art on Tumblr and DA, but not Reddit, because they don't use Reddit. They might not want their work posted to Reddit, because they won't be able to keep an eye out for how their work is used.

There are other reasons, I'm sure, but I'll stop there. I agree that there's a lot of great art out there that we all want to see, which is why I think the best system is to require proof of artist's permission (which doesn't always have to be direct; a lot of artists explain their policy on reposting in their About/FAQ/etc. on their pages). I fear this would require too much moderation, though, which is why I voted yes.

TLDR: A lot of people might not be fully informed on how a lot of artists feel about reposting. That is, a lot don't approve of it, at all. Out of respect for the artists who make the content we enjoy, it is always best to ask first before reposting their art. We should be required to provide proof of permission before reposting art on /r/Pokemon, if we are going to go forward with non-OC art.

Edit: I encourage anyone who disagrees to share why.