For your images, you will need to save a larger version of your image. 600 pixels wide or tall are sufficient for a large image. Then you will need to make a thumbnail image from the larger image using a program to resize. Thumbnails that are125 to 200 in width are usually a good size for most websites. Always work with a COPY of the large image when you resize and save as a thumbnail image. When you have the thumbnail and larger image ready, upload them where it says Thumbnail Image and Large Image when you're adding your product. Those boxes can be found under the description box.
If you need a program to help you with resizing, try the Pixel Resizer. Its free.
Did you know that the website builder will automaticlly creat thumb naail images for you.