Most prioritized, always provides images in original quality. In italic — not 100% sure, but no cases to put it lower. - Aryion; - Inkbunny; - NewGrounds; - NewTumbl; - Patreon (unsure about public posts); - Pillowfort (seems to provide originals, allows 3rd side images embed); - Pixiv (but sometimes fetch receives JPG instead of PNG).
Original image can be obtained, if artist allowed it in any way. Same priority, as previous, but should be checked. - DeviantArt — if download explicitly allowed; - FurAffunity — if any image dimension is larger than 1280 (when image initially uploaded, it's compressed to JPG and 1280x1280 at most, but can be edited); - VK — image can be uploaded as "document", usually parallel upload (click on second image).
Sometimes image is present in original quality, sometimes not. It's not upon artist choice (at least, directly). - DeviantArt — sometimes image can be zoomed in and it will result in original quality; - Twitter — if naturally PNG (url edit does not count!) and if any image dimension is not equal to 1024/2048/4096; - Tumblr — naturally PNG under 1280x1960 for pics with old url format and under 2048x3072 (any format) for newer.
Not explicitly accessible, missing from UI. - Portfolio.commishes — see here ; - Tumblr — see here — for newer pics dimensions in URL can be edited, for older — reblog + dump blog.
Listed roughly in quality order (may be duplicated based on the case): - Twitter — under 1024x1024, 2048x2048 or 4096x4096, JPG if no transparency; - DeviantArt ("exploit") — "intermediary" exploit for older images, under 6MPx, original format — see here; - Tumblr (new image url format) — under 2048x3072, original format; - DeviantArt (""exploit"") — thumb size upon artist choice (under 1920 pixels width), original format if thumb url edited to match — see here; - VK — under 2560x2160, always JPG; - Tumblr (old image url format) — under 1280x1920, can be JPG if image is too large; - DeviantArt — thumb size upon artist choice (under 1920 pixels width), JPG if no transparency; - ych.art — under 1600x1080, crisp zoom, original format; - FurAffunity — under 1280x1280, JPG if no transparency; - Portfolio.commishes — width can be 700, 960, 1920px, can be upscaled, crisp zoom, always indexed PNG; - instagram — somewhat under 1024-1600px, always heavily compressed JPG.