Jack, this is not a support forum, it's a discussion group that is
not really active any more that much. If you had asked on diaspora*
with the hashtag #diaspora, I think you might have gotten a faster
answer.
AFAICT there is no way to "just change" the logo, except by copying
over your own. The best way for running your own pod is to use your
own fork and replace whatever content you need (assets like logos,
etc), then run your pod off your fork. That way you can replace your
things and still have them version controlled.
Br,
Jason