There is typically only one reason why your images are loading slowly and that is image size. High definition or print-ready images have high DPI and therefore contain more information for your computer network to transfer for you to view. When designing and making images for your website, try using image sizes that are established for web use.
For more information on sizing images for the web, try googling "image size standards for web".
Standard image sizes for web: http://www.fileformat.info/tip/web/imagesize.htm