You could purchase an image Viewer/Editor software like Photoshop or Paint Shop Pro, but we recommend a free one called XnView. It's a single file you can place anywhere on your hard drive, and by just loading an image and trying out a few of the options, you can find your way around the program very easily. It can SAVE in several formats, resize, and has many filters like sharpen, etc. - and it's FREE!
Just remember - no matter what software you use, try not to downsize the image TOO much - leave us something to work with. We will modify the file to the correct size and optimization for your site if we are doing the site maintenance.