it can't be as easy as renaming the file

what are the siza restrictions? (and everything else)

well just look at the actually files:
- buildings 200x150
- races 300x300
- ships 200x150
- symbols 200x200
but other files (events, galaxies) are resized into the computer display resolution - maybe all graphics.

png files are looking better when transparency is aktive. (You can also use jpg renaming into png/bop, but jpg doesn't have transparency)

but just try, changed the graphic file, restart BotE (!) ... it's quite easy :-)