I think it's still possible to download raw sources from Tumblr, just very inconvenient. I don't think it's intended so it might be fixed, but with the upcoming ban on adult content I figured it's now or never. If you export a blog you run (Blog Settings -> Export) then you get seemingly full size images even from reblogs; I've received 20MB PNGs and JPGs from this export. I last tested this on Nov 24 as all my blogs are 10+GB and I'm on mobile currently. If there are any Tumblr artists who don't post to a proper art sharing site like Pixiv this may be useful.