A more serious concern is compressibility - a real image file is probably more compressible than a file with the low-order bit replaced by a crypto-bit, since the real data has moderate correlation and the crypto-bits are random. I doubt the Feds will immediately start looking to see if you're shipping GIF files that have significantly worse compression than average, but they'd probably find something if they did it.
how many bits are we talking about here? suppose it's two in sixteen. 7/8 of the compressible bits remain. so if the normal compressibility is 2:1, taking two out of sixteen bits would leave 1.75:1 compression. is that a "notable" difference? i haven't been paying close enough attention -- is two out of sixteen a realistic amount? it seems high to me. if it's one out of sixteen, the effect is only a 6.25% reduction in compression. is that notable? peter