You may notice that sometimes images you upload to the text editor look distorted.  When you resize images in the text editor you are not changing the actual image, just the dimensions that are displayed.  When you have a large image and shrink it in the text editor you are just making the viewable area smaller, not the actual image.  Chances are these images are from a digital camera and are quite large in dimensions for better quality.  The problem is, your website visitors don't need to see a 5mb image file, nor do they want to wait for the image to load when they load the page. 

In order to prevent this from happening, resize your images BEFORE you upload them.  The size you choose depends on what the image will be used for, but an image for the web should never be more than 600-800kb.  There are many tools that will help you resize the images, many of which may already be available on your computer.  Because there are so many tools to accomplish this task we cannot possible give tutorials for each one, but a quick Google search will help you find what you need.