It used to be a simple change of the URL could change it to 1280 pixels, but now it's a bunch of nonsense. What to do?

Go to howto:sites_and_sources_-_n_to_z#tumblr and open up and read the RAW method section there.

You will need to have a Tumblr blog (preferably a secondary blog that you do not intent to actually share posts publicly on) in order to retrieve the original quality images from there, since you will have to reblog the posts with the images you want.

Update: I've also added a bunch of tips to that page that I should've added a while ago.

BooruHitomi said:
Go to howto:sites_and_sources_-_n_to_z#tumblr and open up and read the RAW method section there.

You will need to have a Tumblr blog (preferably a secondary blog that you do not intent to actually share posts publicly on) in order to retrieve the original quality images from there, since you will have to reblog the posts with the images you want.

Update: I've also added a bunch of tips to that page that I should've added a while ago.

Oh good I actually do have a tumblr now that I could use to reblog stuff.

I save all pics on my smartphone and also have blog on tumblr. I like to reblog favorite images and after saved them on my device. Sometimes I use soft for editing pictures.

kamui43 said:
I save all pics on my smartphone and also have blog on tumblr. I like to reblog favorite images and after saved them on my device. Sometimes I use soft for editing pictures.

I think you misunderstood what this thread is about. It's asking how to obtain least compressed and highest resolution file from tumblr for archival purposes, not about how to obtain decent file for personal purposes.

Just grind them through waifu2x a few times.