"Built in" to the OSX firmware is a little gem called "Automator". Incase you weren't aware of this feature, go to your pictures folder and right click a picture.
Go to "more" then "Automator" then "create workflow".
When Automator pops up, click the images folder, then drag "scale images" into the workflow. I personally uncheck "copy" and I uncheck "show workflow when run" so it will only take one click - quick and easy. HOWEVER if you want to resize to any size rather than the size you preset, just click "show workflow" on the scale image action and a box will pop up and let you enter any amount you want to scale it to!
Save as plug in for finder and close.
You now have instant image resizing with a single click. ):
Here's a site that has pics of the process:
Glenn Wolsey : Blog Archive : How-To: Automator Resize Images Plug-In