This is the highest resolution I could find with a working link (https://twitter.com/NIKKAI_CONsan/status/1360578022558949382/photo/1). Should I list that link as the source, even if it's most likely not the original source?

Also, since I very slightly cropped the left edge, would it be considered a third-party edit or should I just tag it as cropped?

Touhou Lost Word staff are more cultured than you think

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Although Meiling and Koakuma kind of dominate the picture, I think Patchy is the real attraction here!

As far as sourcing, I'm not sure where you can find the original at full resolution. Even if you take a screenshot in the game, it's somewhat smaller in the horizontal dimension than this version. The image is used for one of the "Story Cards" in the game:
https://gamepress.gg/lostword/story-card/scarlet-win-white-win
The image on that wiki page is kind of letterboxed and shows there is a good sized band of stuff that has been cropped off around the edges:
https://gamepress.gg/lostword/sites/lostword/files/story_cards/0125_%E2%98%864_Scarlet%20Win%2C%20White%20Win.png

This pic goes hard, feel free to screenshot