A few bit flips in an image are unlikely to be noticeable (it depends on the format though; uncompressed is obviously the most resilient, JPEG should be reasonably resistant too.
Also, I don't think images stay for too long in RAM. Your HDD definitively has ECC.
I've had plenty of corrupt JPGs - they're certainly noticable since a single error tends to mess up an entire macroblock, often even the entire rest of the image.
Was fun finding a bunch of my own photos like that. Luckily that was on a disposable copy and not primary storage, but it was enough to put me completely off non-checksumming filesystems.
Also, I don't think images stay for too long in RAM. Your HDD definitively has ECC.