Serviio

That should work, as there's no special configuration that I am aware of for a NAS device. The ThumbnailsResolution switch can be within a device profile, as well as within Profile #1, so if the profile you're using has that set to DLNA it would override the setting from Profile #1.

I don't use AllowedImageResolutions on my servers. Instead, I edit the ServiioService.exe.vmoptions file located in the \bin folder of the Serviio installation with the following line:

-Dserviio.hdThumbnailResolution=1000

I'm pretty sure that file exists on a NAS install as well. Also, just re-scanning the library won't refresh the thumbnails. Go to the Metadata tab on the Serviio console and click "Rescan Video Metadata". That will refresh the thumbnails.

This topic has been raised several times before see viewtopic.php?f=4&t=17870&p=120772&hilit=artwork+hd#p120772

I have brilliant artwork.
As indicated I do not think it has anything to do with your NAS.
Are you referring to audio or video or both?

If you do not already have a user-profiles.xml file in "Program Files/Serviio/config" directory I have attached one (Generic HD profile) that you can try. That gives you 320x320 by default instead of 160x160) - that's the first step.
Then set the higher resolution you want i.e. 600 or 1000 as you require. e.g. -Dserviio.hdThumbnailResolution=600 in the ServiioService.exe.vmoptions file.
Regenerating the images some times seems to be a problem - experiment by adding a new entry and have patience.
(Note-if you are not using the generic profile for your device you will need to change or setup an alternative HD capable profile).

I have made a request for improvements to this process, particularly as some of the changes are lost on an upgrade. Infuriating. see viewtopic.php?f=3&t=25218#p120942
Also configuration not simple for people not comfortable messing in the innards of Serviio.

That I can't answer. Zip is the one and only developer of Serviio. Since it's not really an exe file, I agree that is seems out of place on a non-Windows device. But it of course doesn't hurt anything being there, either.

for the NAS / Linux, it'd be in the serviio.sh file.

I will try to make this easier in the future versions.