That may be tricky as others say, but by normalizing the start image offset to zero upon saving, the resulting .image *would* be more compressable by the LZ family of algorithm. Whether it is true or not is a question...
Why would it be more compressable? Because there are more zeros in oops?
Presumably, but that's not true at least for GNU Smalltalk images.
2.3.6 (normalizes offset to zero) => 72.4% gzip compression, 76.6% bzip2 2.95d (does not) => 72.2% gzip compression, 76.5% bzip2
(As a side note, that change was made to have faster startup times -- if you don't have to swizzle back object pointers, there is less work to be done on image startup -- and as a prerequisite to implement a shared memory space via copy-on-write).
Paolo