Every digital image size has two measurements. The actual physical dimensions, usually represented in inches or pixels (width by height) and image resolution (or dots per inch (DPI)). Computer screen resolution is 72 DPI, so for web images, this is an appropriate DPI. I would get to the resize menu in MS Paint and type in the dimensions you're looking for and then do a save as. put this file where ever you archive photos and you should be set. Try this and see if it works. Also look at the file size of the original and the new file and see if they are the same or different.
i use photoshop, but that's a way overkill for your needs. i'd imagine paint will do you well if you just get to know the tools available.