An app is using our art without permission

I remember this happening to Rinmaru last year. They spoke alot about it on facebook/deviantart and did end up getting their content removed. It took quite alot of work but it ended up okay in the end.

From what I can see the actual app itself isn't actually hosted on Apple or GooglePlay. The links they provide are actually for the app file itself (apk & ipa) which usually means they hosted the app on their own site to avoid copywrite laws. You still should be able to send a DMCA/takedown. I haven't done this myself but from some google searches, you should be able to do this without too much fuss.

I did a bit of research on the technical aspects of the website (as I can't speak Chinese) and I've found a few things that should help you out.
The useful stuff for you will be; their IP Address/Location, Domain ID, Physical Address/Phone Number & Abuse Contact Information.
The information below should be enough for you to contact the owners through DMCA/takedown or issue the domain owners a notice about the unauthorized material.
whois.domaintools.com/fyhd8.co… <This is information about the site they used to upload the app along with your content.
whois.domaintools.com/baidu.co… < This is information about their linked site. It looks like a forum about the app? thats what I'm getting from google translate at least.

Below are the posts Rinmaru wrote on facebook about their DMCA/Takedown of unauthorized content.

(www.facebook.com/rinmarugames.…
www.facebook.com/rinmarugames.…
www.facebook.com/rinmarugames.…
www.facebook.com/rinmarugames.…)

I hope this helps.
I wish you all the best and hope everything goes well.