import sys
import random

inFilePath = sys.argv[1]
outFilePath = sys.argv[2]
damagePercentage = float(sys.argv[3])

with open(inFilePath, "rb") as f:
    d = f.read()

damageTotalCount = int(len(d) * damagePercentage / 100)

print("Damaging "+str(damageTotalCount)+" bytes.")

dList = [ x.to_bytes(1) for x in d ]

for i in range(damageTotalCount):
    pos = random.randint(2000,len(d)-2)
    dList[pos] = random.randint(0,255).to_bytes(1)
    if (i%1000 == 0):
        print(str(i)+"/"+str(damageTotalCount))

d = b"".join(dList)

with open(outFilePath, "wb") as f:
    f.write(d)

If you run it, the first argument is the input file, the second one is the output file and the third is the percentage of corrupted bytes to inject.

I did spare the first 2000 bytes in the file to get clear of the file header (corruption on a BMP file header can still cause the whole image to be illegible, and this demonstration was about uncompressed vs compressed data, not about resilience of file headers).

I also just noticed when pasting the script that I don't check for double-corrupting the same bytes. At lower damage rates that's not an issue, but for the 95% example, it's actually 61.3% actual corruption.