If you're prepared to spend a little time learning a couple of terminal commands then Imagemagick is capable of doing stuff like this for free, and it's a cross platform standard. If you have a particular set of commands you always use then you can wrap your terminal commands into a bash script, apple script or even with automator.
Alternatively SIPS comes with osx, and although I've never used it I've been told that it provides similar functionality to Imagemagick.
Granted this isn't for everyone, but I highly recommend trying it out. Imagemagick is available from MacUpdate and it's very easy to get started.