Couple things...
http://www.aha-soft.com/ahaview/bmp-to-jpg.htm converts pics enmass to a new extention.
Image Resizer
This PowerToy enables you to resize one or many image files with a right-click.
I downloaded this and at first didnt think anything kicked in... but I tested it out. I went to my picutres area where there was about 200 pictures where the default size is 3264 x 2448 (thanks to my camera settings). I selected all pics, right clicked, told it to shrink the size and BAM! they were all resized FOR me!
schweeeeet!