The image is just a bunch of random gibberish obviously, however we do have the encryption script to reverse. The only encrypting part of the script is the line :

pixel[i] = pixel[i] * key[i] % 251

Since I got lazy I decided to just brute force this as there are only 255 numbers to check.

if ((n * key[i]) % 251 == pixel[i]): pixel[i] = n

If the number n, from 1 - 255 matches the above condition we know the n is the correct number. This will give us the decrypted image.

Original writeup (https://github.com/darkvoid32/CTF-writeups/tree/master/%C3%A5ngstromCTF%202020/Wacko%20Images).