This assumes several things though:

1. That the current poster knows the original source/OP
2. That by posting this, the poster is crediting him/herself as the post's inventor

Point 1 - we have no idea where the current poster first saw this cat. Maybe they saw it on Facebook and hadn't seen the link NoKarma4u posted.

Point 2 - posting something isn't always equivalent of claiming ownership of something - it can simply be sharing.

Reddiquette wants to give credit for work created where it's due - the link NoKarma4U posted is also just another imgur link, not a blog link or anything. That link is not proof (from what I can see in that post anyway) that that is the original picture, so his pointing it out is kind of meaningless. If he posted definitive proof of the picture's origins, it has some meaning. If this picture is an artwork and the artist is losing out on ad revenue or traffic or something, then yes post the original link, but if it's just someone's cat whose picture they stuck on imgur to say "look at my pretty cat!", does it really matter whether we're seeing the original imgur link or the new one, especially when this poster isn't claiming it's their cat or their photo? Besides, NoKarma4U's issue isn't "you're not giving an artist credit", it's PURELY "I have seen this before and so am going to complain about it".

Reddiquette actually specifically says not to complain about reposts.

Don't: Complain about reposts. Just because you have seen it before doesn't mean everyone has. Votes indicate the popularity of a post, so just vote. Keep in mind that linking to previous posts is not automatically a complaint; it is information. Votes indicate how the community values information, so just vote.