How late am I with this tool? Just stumbled upon this through 4chan and used it on a few images I always wanted to use a backgrounds, but didnt possess a high res original, and it's all looking pretty good so far:
It's just a bit tricky to get the noise reduction right I think, if you just go with the highest setting very detailed stuff will look blurry, if you use none it stays a bit more detailed, but gains fragments. (See the magic circle in the top-right corner in the Illya-image).
Though the GuP one seems better with high than med noise reduction, not taking in account the text in the top left corner~
Paint > waifu2x. Seriousness aside, tried it with few pics I used as wallpapers even though they were much smaller than screen's dimensions and it worked quite well. Better than streaching it, at least. It's really fucking good.
I'm really impressed how it takes original which is so damn aliased and pixelated at 100% dimensions which when streched to screen resolution you can count pixels, and make a image out of it that is double the original dimensions AND looks so much better.