optimize/compress screenshots for docbook handbook

(I recently asked this on IRC, but Konversation crashed before I saw  
any response, so apologies if this is actually a duplicate request.)

I'm currently capturing screenshots for the handbook for the new  
version of KMyMoney.  I recall the last time I did this (years ago)  
there was a tool (or more than one?) for optimizing/compressing the png  
files, but I can't currently find any mention of such beyond the simple  
basics of capturing screenshots.

Thanks for any pointers to any fine manual I seem to have missed  
reading.

Jack