The Gimp (Graphic Image Manipulation Program) is a Photoshop level (read that extremely powerful, with a somewhat daunting interface) that is free, downloadable, and works extremely well.
Find Gimp at
http://www.gimp.org/ Make sure you go to the .org site, that's the free version.
Load it up. Open the picture file you want. (Pull down the File Menu, then open)
Then you only need to pull down the Image menu, click on Scale Image. You will get a box to adjust the size of the image. I would suggest a width of 1000 or so pixels for the web. The height box is automatically locked in proportion to the width (you could unlock it, but don't) Enter 1000 in the width box, hit Enter, and your picture is resized. Save this rescaled image; I would suggest you save it as a new name.
Totally free, extremely powerful, way more than you need here, but a wonderful picture editing program.