One of the greatest frustrations for developers using content management systems is spending hours getting a website looking perfect and then handing the reigns over to the client, only to have them do something ... I can think of a few words here but let's go with "unwise".
A fundamental mistake is not resizing images before inserting them into articles as explained in lesson 10 of the Build a Joomla Website tutorials. However if you can't bring yourself to do this, here is another solution using a free extension. Developers take note - this one could save you a lot of frustration.
Before starting, please note that it is always a good idea to make a backup of your site before installing an extension.
2. In Administrator, go to Extensions - Plugin Manager
3. Look for the "Content - Optimizer" extension and enable it.
4. Create an article as normal and insert your image. Specify your desired image dimensions and save the article.
The plugin then takes over by resizing the image automatically and storing it in a new location. This results in a smaller file size which takes much less time to download.