The web is very visual. You may communicate with text, but the images on your website often attract the attention first and may be vital to the goals of your site. You will have a lot of control of the images you use on your site.
You can add images in a couple different ways. You can include them in along with the text, having the text flow along with the images, or you can add them separately and place them wherever you want them to be.
More important than that, is how the builder handles your images. You will probably be including images that you have taken with a digital camera. These images are often at a much higher resolution and size than you will need for your website. In fact, web images are presented as 72ppi regardless of what the image actually is. So uploading a 300ppi image isn't going to look any different, but it will use considerably more storage space and take a lot longer to upload and for your web pages to load.
You can make things much easier by preformatting images before uploading, but the builder will also help to convert your images to a more usable setting. Then, just like text boxes on your website layout, you can actually stretch and shrink the size of your images right in the builder, so you don't have to keep going back and forth creating different sized images for your site.
Here's the really important part. Often what happens is that the browser is just given the same image, but given different width and height attributes, which results in a distorted and lower quality image. Site by Nite actually resamples the images in the builder, which means the quality and look of your images is maintained even after you've changed the size.