When I resize images to post or to use in my avatar I use Photoshop. Any image program will work. Once you have the pic. up that you want to resize, you would go to "Image" at the top, click that. Then there should be something that says "Resize" or "Image Size". Scroll down to this, then click. If there's a box that says "Constrain Proportions" after you've clicked on "Resize" or "Image Size" check that box. This allows the pic. to be resized properly. Meaning, if this box weren't checked then you would have to type the Height and Width manually.
Sorry for being so long. Just wanted to be thorough with the details. Hope this helps.