stsyn 
Moderator
Idea is to put all knowledge about sites ("1st place" sources — not boorus) behavior in one single place. It may help you to understand, which sites tend to provide better quality pics.
ALWAYS ORIGINAL
Most prioritized, always provides images in original quality. In italic — not 100% sure, but no cases to put it lower.
- Aryion;
- Inkbunny (but be sure to upload not a thumbnail!);
- 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).
—————————————————
UNDER CERTAIN CONDITIONS
Upon artist choice
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 (requires login and fetch does not work in this case!);
- 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).
Reasonable technical limitations
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.
"Exploits" exists
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.
—————————————————
NOT ORIGINAL IMAGES
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.
ALWAYS ORIGINAL
Most prioritized, always provides images in original quality. In italic — not 100% sure, but no cases to put it lower.
- Aryion;
- Inkbunny (but be sure to upload not a thumbnail!);
- 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).
—————————————————
UNDER CERTAIN CONDITIONS
Upon artist choice
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 (requires login and fetch does not work in this case!);
- 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).
Reasonable technical limitations
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.
"Exploits" exists
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.
—————————————————
NOT ORIGINAL IMAGES
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.