Unfortunately a solution to the problem is constrained by Australian copyright law.
To give you some background on IceTV so you understand my reticence...
From about April 2006 IceTV fought a 3.5 year battle against Channel NINE after they sued IceTV whilst IceTV was in the middle of a capital raising to list on the ASX. Their grounds were that we breached copyright by reproducing various parts of the EPG that was produced by their subsidiary HWW Pty Ltd. Google it and you'll find the details. We eventually won with a unanimous decision in the Australian High Court. Nevertheless, as in most commercial litigation, the victory was somewhat phyric as only about half costs are recovered and IceTV had been denied the capital it intended to raise by the IPO, so was starved of funds going forward.
The use of 'thumbnails' is not necessarily permitted unless the content owner, who has produced the 'thumbnail', gives permission to the broadcaster for their use in the broadcaster's promotions, and those uses will be constrained by the relevant contract(s). The 'thumbnail' is undoubtedly copyright as it has an 'author' who would have designed it whilst in the employ of the studio or content owner.
In the USA there is a section of their copyright law called 'Fair Use' which may, or may not, permit the free use of these 'thumbnails' by the likes Emby, Kodi, Plex and so forth. But it may be they just get away with it as the owner is happy for the promotion of the work to be done for free. But that doesn't mean the law is not being broken. I am aware that the studios in the USA are looking closely at Kodi due to it's prevalence in the replay of pirated movies.
In Australia our similar law is called "Fair Dealing' but it has a higher threshold of permitted use and our lawyers have stated that we cannot just take those 'thumbnails' without permission, which we would never get. Just as we couldn't get access to the EPG metadata and had to find ways to construct it ourselves and fight a battle to the High Court to prove we were entitled to do it as we do.
So what we do for our own 'thumbnails' is to take snippets of the broadcasted show which is an insubstantial image in copyright law and use those for 'thumbnails'. We can't use the proper promotional 'thumbnail as that is a unique piece of work and copyright in itself.
If we were to try to 'push the boundaries' and use the proper promotional image(s) I am certain that the long memories at the various commercial TV stations, smarting at their loss to my little company, would quickly seize the opportunity to sue us again and this time with far more certainty of winning.
As for the use by Emby, and others, in Australia I'm assuming they are just getting away with it as they are difficult targets to sue from here when their actual operations are based overseas.
I hope that answers your questions, and similar ones that get asked by others at various times.