1. 1.
    When we view the source of the website, we can look at the stylesheet at /style.css, and see that it loads an image from concat_v.png. Let's download this image.
  2. 2.
    Since this image is a PNG, we can use a steganography tool called zsteg like so: zsteg concat_v.png
    zsteg concat_v.png output:
    imagedata .. text: "\n\n\n\n\n\n\t\t"
    b1,b,lsb,xy .. text: "picoCTF{imag3_m4n1pul4t10n_sl4p5}\n"
    b1,bgr,lsb,xy .. <wbStego size=9706075, data="\xB6\xAD\xB6}\xDB\xB2lR\x7F\xDF\x86\xB7c\xFC\xFF\xBF\x02Zr\x8E\xE2Z\x12\xD8q\xE5&MJ-X:\xB5\xBF\xF7\x7F\xDB\xDFI\bm\xDB\xDB\x80m\x00\x00\x00\xB6m\xDB\xDB\xB6\x00\x00\x00\xB6\xB6\x00m\xDB\x12\x12m\xDB\xDB\x00\x00\x00\x00\x00\xB6m\xDB\x00\xB6\x00\x00\x00\xDB\xB6mm\xDB\xB6\xB6\x00\x00\x00\x00\x00m\xDB", even=true, mix=true, controlbyte="[">
    b2,r,lsb,xy .. file: SoftQuad DESC or font file binary
    b2,r,msb,xy .. file: VISX image file
    b2,g,lsb,xy .. file: VISX image file
    b2,g,msb,xy .. file: SoftQuad DESC or font file binary - version 15722
    b2,b,msb,xy .. text: "[email protected]"
    b4,r,lsb,xy .. text: "\"\"\"\"\"#4D"
    b4,r,msb,xy .. text: "wwww3333"
    b4,g,lsb,xy .. text: "wewwwwvUS"
    b4,g,msb,xy .. text: "\"\"\"\"DDDD"
    b4,b,lsb,xy .. text: "vdUeVwweDFw"
    b4,b,msb,xy .. text: "UUYYUUUUUUUU"
    As you can see, the flag is shown in the output.
  3. 3.
    More information about different steganography tools can be found on HackTricks Stego Tricks