Making a patchfile

Hi all,

 the situation:
  I have a checked out copy of a repository of an open source project
which is publically readable
  I don't have commit access, nor do I want it, nor would I expect to
get it
  I have made improvements in my working copy
  I want to send a patch to the project in question for them to examine
and if they want apply my changes
  
How can I generate said patch?

 svn diff >patch.txt
Won't cut it because my changes include removing and adding various
binary files (images).

How is this normally done?
  

-- 
James Sleeman <james@gogo.co.nz>