Specifically concerning skins, Riot's basically said that they don't have the art resources to update every champion's skins at the same time as they get a visual update. Since they're now a low priority, Riot made the decision a couple months back to use the Chinese splash art for some skins (which you may have noticed). They'll be updated eventually, but not until then.
Meanwhile, champion splash updates operate slightly different. Lemme throw in a quote from RiotSilver, since he does a better job of explaining it than I do:
"Hey guys,
>Just a heads up, because I'm seeing a lot of comments wondering why we picked certain splashes to update. We are working on updating a large number of base champion splashes in tandem and integrating them into the game as soon as they become final.
Some splashes move through the pipeline more smoothly than others based on a number of different factors including feedback and resource allocation. A number of the splashes you guys are asking for are in the process of being finalized and we will be integrating them into the game the minute they become final. A few just went in and will probably show up on PBE tomorrow.
>What this means is that even though splashes like Rengar have been updated does not mean that we are overlooking splashes like Anivia, Shen, Singed, Maokai, Udyr, Etc... It just means that Rengar was finished faster than them for one reason or another.
>The illustration team here is hyper focused on finalizing these splashes as soon as we can and super stoked to be able to show them soon. As always, continue to post feedback and I will do my best to check back in.
>-RiotSilver"
So they'll get to Anivia and Warwick eventually. Anivia's I think is already fairly well along in production (it got spoiled during Worlds) and Warwick's likely won't come around until he gets his long-awaited gameplay update.
