xv is available in virtually any Unix platform and downloadable from the web.
xv (one of the unsupported programs supplied by SGI) can do that with
the autocrop option.
ipaste is a simple SGI program which can also be used.
For more sophisticated applications, however, look at
and for even more, get from most Linux distributions or download:
I hope this small survey might be usefull ... Would be good to
have some more hints for good graphics software posted ... (Morten ?)