I was waiting for a post like that and i am really happy to see it:) i am not a painter but a collage artist and the problems with copyrights are much more complicated in this medium. Still if someone is careful and sensitive as an artist he can express himself in a legit way.
Its been a year since i focused more in curation and finding valuable art in the platform and i saw many examples of artists copy or change a bit another work of art to make a new one of their own. I voted most of them, i do not have a problem if someone sees a photo and make a painting out of it and he has no obligation to attach the ref image at his post. This is how the painters work, they see and they create. Even if they used their imagination, the image in their minds is sth they saw in their life and the viewer appreciates the technique, skill and sensitivity of the artist. None cares if the model was a photo from an ad of a jewellery shop and it is copyrighted.
When this Anti Plagiarism Crusade started i commented a lot and warned that this will destroy the art at the platform. Not only the artists accused will stop posting but also other artists who wittness this behaviour of sticking the label of plagiarist to another fellow artist will be disgusted and stop also. Not to mention, as you rightly said, that we can not expect new established artist to come and start posting here.
To conclude, i strongly believe that there should be a more sensite approach to this matter that would involve discreet guidance and private communication.
Yes, we need to welcome new people and support the ones who are here, not run people off with made up arbitrary guidelines.