I used Handbrake on my Mac and re-encoded the MKV to MP4. From there I used a some freeware - Subler (awesome program) to import metadata (gives you a movie description and attaches it to the file). After you add this you choose your artwork (provides many options) and it attaches it to the file. Hit save. Hit optimize. Hit Send To iTunes. Then it will be syncable with your iPhone thru iTunes and will place it in your Videos app along with all your other movies/purchases.
I have SW 2.5, ESB 2.0 and ROTJ 2.5 all on my iPhone 6 using the steps above and they look absolutely incredible. I may throw together a YouTube tutorial if it would be helpful to ppl.
EDIT: also off topic. My apologies. If you want more info PM me