This problem requires you to install zsteg, a common CTF steganography tool to obtain the flag
$ wget http://mercury.picoctf.net:48380/concat_v.png
$ sudo apt install ruby
$ gem install zsteg
$ zsteg concat_v.png
picoCTF{imag3_m4npil4t10n_sl4p5}picoCTF{imag3_m4npil4t10n_sl4p5}