Wacko Images
We have ab mod c = d where we know b (key), c (251), and d (encrypted pixel value). Multiplying by the modular inverse of b gets us: a mod c = d * b^-1 mod c
Calculate this for every pixel in the image with a script, and you get an image that contains the flag.
The flag is:
actf{m0dd1ing_sk1llz}
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