Your experience with online community seem to be awfully specific XD.
I understand where you're coming from, I kinda lived a similar thing, but it seems we reached different conclusions.
First I suspect that most people ( that could include me as well) have no real idea of what a good art is anyway. I mean i would have much lower expectations than a skilled artist for sure.
However there are basic reasons people tend to defend hard work (or even easy work) for this kind of things :
1) The vast majority of people have no idea how to do it themselves (even for simple things as portraits). Some even can't follow simple instructions on how to apply mods.
2) New moders need support to persist, and crushing them can just push them out.
3) veteran moders can aim to improve themselves thus making you kinda invest into them.
For the record, I fall on the 2nd category. I remember making my first and final mod for pillars of eternity 2 a few years ago (my first playthrough) tweaking the monk gameplay and to go along with it I put a portrait found on the net. I contacted the picture author for rights and mentioned that I did not manage to reach the person yet and would remove the picture if necessary. After a few days I had much positive feedback on this because the tweak were wanted for a while from many people. Unfortunately I was denounced for the picture usage, my whole post was banned from nexus who just linked to me my own words "I haven't reached the picture author and will remove it if necessary", and asked me to remove the art.
Since I found it unfair and brutal, mostly on how communication went, and also knowing that 80% of the art found in others mods weren't even concerned with the issue at all, they just didn't mentioned they didn't have the right, I decided to remove the mod entirely from nexus mod, use it only for my personnal usage, and never look back on the modding scene.
All this to say we can't really control online forums, would be a waste of time anyway. Their choice if they don't want your art, my tweaks, or anything proposed to anyone.
But I don't intend to reduce anyone's work for that matter, be it small or big. I'm not sure if you worked on those pictures or if you found it on the net as is, but even the work of finding those is no simple matter for may people and must be acknowledged.
In any case, the art you provided to me is vastly improving my gaming experience.