As I was using Photoshop to resize my eleven billionth image, it occurred to me I should probably simplify this process. AppleScript, perhaps? Photoshop scripts? Some already existing little app? A little pontification on Twitter resulted in a number of suggestions to try OS X's Automator. Turns out it's a pretty easy thing to do, at least in a limited way.
Open Automator, then...
1. Make a Service
This is really up to you. You might prefer an app that you can drag things onto. Or a hot folder you can just drop things into. I like services since I can use them from the context menu anywhere.
2. Add "Scale Images" Action
It's under "Photos".
It will ask if you want to make a copy of the image or not. You probably do, so you can keep the original in case it doesn't turn out how you want.
3. Add "Open Finder Items" Action
Open it with ImageOptim. In my case I'm scaling the images for the web, so of course I want them to be as optimized as they can.
4. Save It
I like calling it something super obvious (to me), like simply "500" or whatever the max size of the scaling is.
5. Done! Now it's available as a service for any file in the Finder no matter where it is.
This is pretty rudimentary
It has zero logic. So if you drop a 437px image into a service that scales to 1000px, it'll scale up which you almost certainly don't want.
It keeps whatever format it's currently in. Perhaps your PNG makes more sense as a JPG, this can't help.
It not always the greatest quality. I might look into attempting an Acorn or Photoshop thing instead since they seem to keep quality higher.
Would love to hear about your photo resizing workflows.