First, go to your Listing Dashboard. If you need help locating your Listing Dashboard, see this article.
From the overview page, you can download the images from these two location. Note: The bottom left button will be visible from any tab.
Once you go to the click on the "Download Photos" button you will get a prompt to select your file size. You can download the images in web, MLS-specific sized images, print or download all sizes. If you need to change your MLS provider, click on the "change MLS" and this will allow you to download image sizes optimized for that MLS service.