The best way to add an image is to size it in Photoshop or another image software then import it. If that is not possible, you can resize and crop it still.
To resize you will upload the photo to the media library. When you resize you will keep the aspect ratio so the photo is not distorted. What you will do is try and resize it to the correct pixelxpixel size. If it is not possible use the ratio (if the photo needs to be 1600x600 and you cannot get the exact dimensions make sure the ration is 8:3). This will make your photo fit.
If it is still too large you can then crop it to fit. This will limit how much of the photo appears, but it will fit and look great!