Log into your cpanel and scroll down to the bottom of the page. Under the Advanced menu, you will find an Image Manager link. The second link, Image Scaler, will allow you to change the size of your images. To find your images, click on the public_html folder and then on the images folder. Scroll down to find the product image you are looking for and click on the image name.
When resizing, make sure you keep the 'Keep Aspect Ratio' box checked. This will make sure that your image doesn't get distorted. Also, keep in mind that your visitors will have different screen resolutions. Common screen resolutions are: 640 x 480, 800 x 600, 1024 x 768, etc. So, you will want to keep your images within these dimensions if you don't want your visitors to get those scroll bars.
Note: the 'Retain a copy of the old image as' option will take up more disk space, so use this at your own discretion.