Problem: 🥛 (http://mercury.picoctf.net:48380/)
File: THE_FILE
Solution:
Going to the site you can see an image of a man getting slapped with milk when you move your mouse.
Looking at the js and css you can see how it works. Looking at http://mercury.picoctf.net:48380/concat_v.png
This is the large image they use to make it work.
Checking the file
binwalk concat_v.png
strings concat_v.png | grep pico
exiv2 concat_v.png
zsteg concat_v.png
Flag: picoCTF{imag3_m4n1pul4t10n_sl4p5}