We can't see the data but it depends on three things
First, the format of BMP files, which may not be what you expect:
BMP file format - Wikipedia[
^] - it can contain a whole load more than just the pixel data, and all of that is are the "front" of the file - so the actual pixel bits are at the end, not the start. It's also very possible that changing a pixel will cause additions or subtractions to parts of the header info.
Second, Base64 is not an encoding algorithm - it's a translation algorithm which "spreads" the value of bytes across multiple output characters: so a change in a pixel value can alter several values in the output.
Thirdly, if you start randomly changing values in an actual file (rather than the Image data that most apps work with) you can easily render the file no longer readable as a valid BMP file at all - and it doesn't take big changes to do that! Think of it as your bank card number: if you change one bit in that then one of two things happen: your bank card either accesses the wrong account (very unlikely) or your card s rejected as the number is no longer valid (bank card numbers include error checking to ensure that it's really difficult for small changes to go unnoticed).
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