boorus
Most booru-type websites will show you a low-res image first. Sometimes you get a notification, sometimes not. Look for a "download" or "view original" link, usually on the left side toward the bottom. If the link, path, or filename has "sample" anywhere in it, that's not what you want.
Example URL (bleachbooru):
smaller:
https://bleachbooru.org/data/sample/17/b0/sample-17b09fa41ebe7b2d38f727055963726a.jpg
best:
https://bleachbooru.org/data/original/17/b0/17b09fa41ebe7b2d38f727055963726a.png
Example URL (e621):
smaller:
https://static1.e621.net/data/sample/13/02/1302f899e609b1ebb87792a085c9dd67.jpg
best:
https://static1.e621.net/data/13/02/1302f899e609b1ebb87792a085c9dd67.png
Baraag
Baraag shrinks large images, and compresses high-quality JPEG, though the exact thresholds at which these happen is unknown. Try to find another source, especially for JPEG and for images that reach 1200 pixels in either direction.
That said, for images on Baraag you should click the image in the post to enter the single-image view, then get the link for that image. If you did it correctly, the link should contain "original" and not "small".
Example URL:
small:
https://media.baraag.net/media_attachments/files/110/696/342/337/813/656/small/956a8a183cefecfa.png
better:
https://media.baraag.net/media_attachments/files/110/696/342/337/813/656/original/956a8a183cefecfa.png
deviantArt
On dA the highest-res version is only available to the artist unless the artist specifically chooses to make it available. Thus, the best advice here may simply be to try another site. That said, be sure to click the "Download" button on the right, since that's usually a better version. If there's no such button, and you can't find another source, click the image.
Discord
Remove the width and height arguments from the end of the address, so that the last part of the address is just the filename.
Example URL:
smaller:
https://media.discordapp.net/attachments/786827899269152768/797901581151633428/Screenshot_42.png?width=1202&height=676
best:
https://media.discordapp.net/attachments/786827899269152768/797901581151633428/Screenshot_42.png
e-hentai/Sad Panda
If there is a "Download original" link below the image, click that. Note that during "peak hours" this option may be unavailable to you, or cost you a "currency" like site credits. You can also download a gallery archive, or a torrent if one is available, but make sure it is original resolution/size and not, for example, a 1280x resample.
If there is no "Download original" link, the image file shown in the gallery should be fine.
kemono.su
Click the image. View it directly and make sure the path/link doesn't say "thumbnail".
FurAffinity
FA resizes and compresses images larger than 1280 pixels on either dimension. Worse, sometimes it doesn't change the filetype to JPEG even though it has compression artifacts. If the image actually reaches 1280 pixels on either dimension, you may wish to check another source.
[Expand Post]
Click the "Full View" link below the image to be sure you're getting the full size. It should be pretty obvious if you aren't, though.
HentaiFoundry
HentaiFoundry sometimes resizes images that are over 1000 pixels on either dimension.
Use the "download" button. If there isn't one, click the image. View it directly and make sure the path/link doesn't say "thumb".
imgur
Don't use the mobile site; we are unaware of any way to get originals except on the desktop site. On there, check the "..." or "meatballs" menu for the "download" option.
Inkbunny
Click the image, or the "Download" link below it. Don't directly view the sample image. If you did it right, your file path should have "/full/" somewhere in it; if not, find "/screen/" and replace it with "/full/".
Newgrounds
Though a good source for the first image in any post, subsequent images in a post are likely to be shrunken and/or compressed. Look for another source when a post has multiple images.
Patreon
Click the image to bring up the full-size version.
Pillowfort
The image should be a link to its full-size version. Click through, or right-click and select "Copy Link Location" and not "Copy Image Location". File name should not have "_small" near the end of it.
Example URL:
small: https://img3.pillowfort.social/posts/86a478cb9e29_snake_princess_small.png
best: https://img3.pillowfort.social/posts/86a478cb9e29_snake_princess.png
pixiv
Art on Pixiv is often censored; if it is censored, you may want to be sure it's not available uncensored elsewhere. Otherwise, Pixiv is a decent source. Note that one entry on the works page may be a multiple-image gallery, with alternate versions including an uncensored version buried within.
For still images, click the image in the post, this will enlarge the image after that right click and save it.
If it's a collection of images, be sure to click the "show all button" and click the images.
When you view the image directly, the path should say "/img-original/", not "/img-master/", and the filename should not have "_master1200" near the end of it. If it does, replace "/img-original/" with "/img-master/", and remove the "_master1200".
Example URL:
small: https://i.pximg.net/img-master/img/2016/09/11/10/48/24/58923402_p0_master1200.jpg
best: https://i.pximg.net/img-original/img/2016/09/11/10/48/24/58923402_p0.jpg
For animations/ugoira use: Powerful Pixiv Downloader or PixivUtil2, you may also find some online tools that convert ugora URL's into a gif or webm
Twitter / X
View the image directly, then edit the filename and make sure it ends in ":orig". That should always work, but if not, try ":large". You may get redirected, but if the result ends in "orig" it should be okay.
Example URL:
small: https://pbs.twimg.com/media/C_rlK-cXYAAh27U.jpg
better: https://pbs.twimg.com/media/C_rlK-cXYAAh27U.jpg:large
best: https://pbs.twimg.com/media/C_rlK-cXYAAh27U.jpg:orig