Great "Tutorial" for Newbees. But also a reminder for us old dogs.
There is one problem I have however with really old posts: Like, I linked to some on my website (on the respective artwork page) but find that for many, Steemit does not show the images anymore, and besides that, I am not really happy with the posts I had written back then about some of my major work.
I did bring some up again in an altered form, such as in connection with a show, or perhaps to discuss details and methods a bit more than in my original post from years ago (whenever possible, I add the link to the old post).

As for quoting: I put them into quote marks. Usually I lead with a linked title (from a newspaper, for example) and add my own comments under the quotes (particularly when I disagree with what is written, lol).
For some images, such as frequently shared memes, it is hard to credit a source, since for those, original sources are mostly unknown, and just linking to one where I got the image from does not really make sense, since they probably stole it from somewhere else (so why should I credit a thief?) - so I usually just say <source: meme found on Facebook>.

Having an account on Pexels I sometimes use images from there, but I make it a point to credit the artist, same with any other CC. What I find with photos I had posted on Flickr (with a pro account I get monthly updates from Pixsy) it is interesting to see how many shots I took that are surprisingly similar to others: for example, I constantly get Pixsy updates about a artwork I photographed at the Moreau Museum in Paris. The same also with many famous landmarks. A good thing that the debate about Panorama Freedom at the EU Parliament didn't result in another stupid law. But that is different in many countries in the EU and also around the world.
Would I have to edit out the Montmartre in a selfie of mine from Paris?
On the opposite of this is the Rijksmuseum, which provides high-resolution images of many of their artworks and even encourages you to use them in applications - check out mine here, on the Rijksstudio site: Otto Rapp Creations - Rijksstudio