The image is encrypted by using modular multiplication with three key
bytes.  It's possible to undo this by multiplying with the modular
inverse.  Given a suitable modulus, the modular inverse for `pixel *
key % prime` is equivalent to `pow(key, prime-2, prime)`.

The solve script has been created by replacing the input/output files
and each key byte with its modular inverse equivalent, using it
results in an image spelling out the following:

    actf{m0dd1ng_sk1llz}