Photoshop is what I use. I think there are some online ways to do it too, or if you use the one here on BYC, there is a way to shrink them when you upload them if I remember right.
The free image hosting things online ..photobucket and imageshack, etc. ...those have resizing features also. Some Windows products have their own built in, as well ...just right click on a pic in your My Pictures and if you have it, there's an option there in the drop down display.