Access pixels without loading the complete file into memory

[anthony]

Not with IM. Also wether you can actually do it at all depends on the IM format.

JPG images will probably not be simple, due to its speciallised compression,
but if it is posible, remember the pixels may not be exactly the same colors as what you had when you saved the image. JPG is a lossy compression and purposaly losses information (even at 100% quality) to allow the image to compress better (hopfully so that the change is not eye visible).