Source notes:

Baraag.net is the best source for images, as they are neither censored or compressed. (replaces Pillowfort since take-down)

Pixiv has original-resolution images in PNGs, but censored.
Tumblr has PNGs, but currently only images ≤1280px can be obtained from Tumblr. ("_raw" URL exploit was fixed)
Twitter has original-resolution images, but compressed into JPG. (Append "orig" to image URLs)

Safe-mode blocked URLs:
https://youjomodoki.tumblr.com