Anyone could upload images at the full res. It's perfectly doable, but getting them varies with each site. Twitter is the easiest by simply adding ":orig" at the end of the image url. It makes a big difference between not including :orig and including :orig. Even if the size doesn't change, add :orig anyway to make it into a habit.

In Pixiv, you gotta have an account, and then on an image's page, you click on the image and either save it or copy its url to get the full size. Copying the url as is gets you the preview size. If it ends in "master1200," that's the preview size. For multi-image posts, there are these two arrows on the top left corner of each image that would open up the image at a new page at its full size. Copying the image url as is would also net the "master1200" preview size.

DeviantArt posts sometimes have a download button to get the image at it's full res. It's at the right on a desktop, and on the bottom of the post on mobile. It's a genuine download button and not some fake download button found on torrent sites. If it's not present, either click on the image to get it as large as you can get it, or see if the artist posted it elsewhere. Boorus normally have a resize option at the top of the image and clicking on that resizes the image to its full size.