Because this factor into deciding whether or not a user comfortable visiting your website
For instance when making a website gallery.
The image file is usually made with high resolution and which create heavy loading process. This could slow down the web page. To reduce the burden of loading, when a visitor visits your page, simply view only the thumbnail of the image. When the user clicks on the thumbnail, with a touch of AJAX, we can display the original size of the image. This technique is quite effective to reduce the process
Take advantage of the cache feature for the website, the popular CMS like WordPress plugin features the cache, the cache on a sample plugin for wordpress is: WP-Super-Cache, which can convert dynamic pages into static HTML files to reduce the use of server resources.
If you use WordPress, you can try this plugin.
Because of the typically slow access is a picture, then before the upload is better at first reduce the resolution.
Use image processing software. Also use the appropriate file format, with the proper file format for images, you can optimize the size without having to lower the image quality. For example, use the JPG format for photographic images, and PNG format for graphs, diagrams, etc.,
with a transparent effect to save the image size.