Image suddenly altered

IMHO, that behaviour depends on two things:

1) Your CSS might be truncating the image by setting a max-width somewhere on the container (I happened to come across that issue lots of times doing my own testing!)
2) You might be using Jetpack’s Photon or a similar mechanism to truncate/resize the image for mobile users (smartphones/tablets), which might also truncate it on desktops in some cases. The imagemap tag is pure HTML (quite old HTML, too!) and will not work on resized images at all (but might work on the non-truncated part of the image).