Great example to familiarize yourself with automator and make this super easy. Automator is your friend!
Open the Automator editor [Applications]
Drag the following three actions from the left pane to the right pane in this order...
"Get Selected Finder Items" (under Files and Folders)
" Scale Images" (under Photos)
" Copy Finder Items" (under Files and Folders
In the Scale Images box that is now on the right, set the image size you want for you output.
Save the automator file to your desktop.
You've now written an automator script that will resize any image you drag to that icon and it will spit it out the resized image on your desktop. Drag images in directly from iPhoto, the desktop, finder, etc.