piespy said:
I see a lot of posts where a pixiv image URL is in the source field. Are those uploaded directly from Pixiv? Whenever I try this, I get an error, presumably because pixiv returns 403 without the correct referrer header.
So do people just paste the URL afterwards or is there a trick?

Yes, they are directly uploaded from Pixiv. AFAIK, you need to be logged in to be able to upload properly with the source URL. I have no idea how you can actually see the pic if your problem is a referrer error.

piespy said:
Also, since one can't simply open the pic from the URL, wouldn't it be better to paste the URL of the page showing the pic instead?

Pixiv stores the images in the same name as the artist's user name. I think this can help with the artist name detection more that the actual source page which only shows the image ID in the URL.