I use this tool: Imageresizer from Codeplex.com
It's a totally free utility under GNU-lisence. It works flawlessly under win8 too. It resizes single pictures to batches of well over a hundred at once. I routiniously resize well over 100 screenshots in under 5 seconds with it.
Image Resizer is fast, makes your screens extremely light-weight which is totally what you want for what you're doing and you have almost no fidelity-loss.
for the gamebooster a quick registration is needed, but personal details are not asked.
Here's the linkage: Gamebooster
A quick note to inform you all that the image resizer that I use has been greatly updated and has moved hosts. The tool can now freely be had from this link:http://www.bricelam.net/ImageResizer/