Here is the best program to use...i use it all the time
Batch Image Resizer - i use this for all my web galleries that i do to get smaller images then another program to auto make the galleries.
has a bunch of free ones as well.
They can do whole folders and stuff really quickly so you don't have to use them all at once.
just do a search for "image resizer" and you will get a lot of different ones.