This seems likely to be unsuitable for large files (e.g. 50000x50000 pixels) because the external compression is going to make random access difficult. So extracting or displaying a small part of the image means you have to read from the beginning.
You need to use a compression format that has sync points. For example, if you need to seek to 1GB in the uncompressed file, the compression itself needs to have recorded where that corresponds to in the compressed file.
zlib apparently supports the `Z_FULL_FLUSH` sync option which you can use at compression time to provide sync points. (I haven't used it, and it doesn't seem to be exposed by GNU gzip.) Other compression formats might have something similar.
