I have a very important Inspiration folder into which I save everything that catches my eye in the hopes that they'll spark some creativity at some point. While I'd prefer everything in there to be high-quality, my rule really is just anything that sparks my brain, so there's some pictures that are quite low-res.
So when I heard about letsenhance.io today, which claims to use machine learning to up-res images up to 4K, I knew exactly what I wanted to spend my 5 free credits on. The results are pretty good, I gotta say. I recommend clicking/tapping to go into gallery mode (plus I just implemented photo galleries on here).

Image credits:
1) thvndermag.tumblr.com
2) crusaderarts.tumblr.com
3) r2-r.tumblr.com
4) twitter.com/cIoudfuI
5) twitter.com/kounnaju