Photoshop is by far the best program for numerous things. They also have
Photoshop Elements which is cheaper in cost (
if you plan on actually purchasing). Depending on what you are looking to do with the software should guide your decision. Dont fork out the cash for something just cause everyone here says you should get it.
There are free utilities online that will resize images, save for web, change colors, hues, saturations, create b/w, sephia, etc... here are a few i found.
http://www.freewarehome.com/Graphics/Graphic_Manipulation/Image_Resizing_t.html
That site has a ton of free software with what each one can do.. id say start with that and if you want more out of it.. try this
http://www.vicman.net/vcwphoto/
also free..
hope this helped you