No, not always, but usually
There is a bit in the header, which indicates, whether it is up or down.
I once wrote a program to convert Zeddy Hires to BMP and set this bit different than usually. Thus I did not need to mirror the HIRES picture, but the PC programs (Firefox or similar) show the BMP correctly even when it is not upside down
Siggi
My ZX81 web-server: online since 2007, running since dec. 2020 using ZeddyNet hardware http://zx81.ddns.net/ZxTeaM