photo apps for ipad over wifi
If your nas supports DLNA then that opens out great possibility's. Just write DNLA photo any you will find plenty of Apps otherwise you will be stuck with NAS unfortunately i dont have any recommendation for nas connection. 

This is what i recommend.. 

https://itunes.apple.com/bh/app/smartstor-fusion-stream-dlna/id410280827?mt=8

https://itunes.apple.com/bh/app/imediashare-lite-video-on-tv/id302026401?mt=8