Maybe I'm slow on the pickup but I just tried something and it worked and it was really quite satisfying to see.
I had taken a bunch of pictures with my digital camera, and imported them, and they came in at over 4000 pixels wide, 3.3mb each. (all jpeg) Too big to post online. I wanted to resize them to 800x600'ish. Normally I'd use GraphicConverter but it's getting superbuggy with my os, it's way old.
I found Preview had a Size menu option for this and it worked well, but there wasn't even a hotkey for it. So... one at a time I am changing them....
Then it hits me... select all. size. 800. return. ding ding ding ding
oh.. my... THAT was slick.