I tried various, (Universal Media Server, Plex and minidlna) and none seemed to send thumbnails fit to the requirements of the software, I always could see "ffmpeg -ss blahblah" on my server in, say, top (Linux and MacOS). I have to say that dlna implementation lacks thumbnails/cover art on the whole platform. The only software being able to show cover art in any case being Samsung Browser (!).
I saw there are slight differences in dlna to what is being sent as "preview" to the files : cover.art, album.art, thumbnail (actual preview), and an other I can't remember, and that what is served is (if possible) what is asked explicitely by the client. If not explicitely asked, the server defaults to none. The only server providing all types of previews I came to was Plex. But as it builds very large previews and all, this is where things get tricky : not all clients understand what is sent.
I looked at the thumbnails that SkyBox provides both on my mac and on the go and saw they are all the same size, that size I guess, being the size of the video elements in the grid of the browser on SkyBox. As it is a fixed grid (still guessing), maybe this is why servers rarely send conformant responses, as, this being implemented by the developers of the software, some software use jpg, other png (as supposed, most do), some keep the original ratio, others shrink or crop to a constrained size, for short, I understand that being compliant to any server is a hell of flexibility and you're better off building your own, to be sure they are displayed and fit in the grid...
One last thing, when building thumbnails for large folders, along the list, some of the thumbs get mixed. Some files get thumbnails from other files, which thumbnails are correct, so some thumbs are repeated along the list, and the only way to force SkyBox to recreate them is to purge the cache folder and reboot the device. I am not sure of that but I think this happens if I browse to some other directory while the thumbnailing is done in that previous folder. However, interestingly, you never get a thumbnail from another file in another folder.
I am sorry I feel like I'm complaining, this is not the case, even if it looks alike. You're doing impressive work.