The whole idea of a Content Management System, is that once the site has been built, and you have been given training in its operation, you start changing content, adding images and new pages.
The hard part is being able to get images to fit and to reduce their file size. Huge images inserted into a page, will make the page load very slowly. This is definitely not something any website owner would want.
Luckily there are many free image editors and many free image editor plugins for WordPress. One such program for desktops is a free program, Irfanview.
Matt Cromwell has already written a great post on this, so if this is something your interested in, please click here.
GIMP is another free image editing software, click here to download.
Compress JPEG & PNG images is a free image editor for WordPress plugin that automatically optimizes your images by integrating with the popular image compression services TinyJPG and TinyPNG. Free for approximately 100 images per month. Click here to download, Compress JPEG & PNG images plug-in
The EWWW Image Optimizer is also a a free image editor for WordPress plugin that will automatically optimize your images as you upload them to your blog. It can optimize the images that you have already uploaded, convert your images automatically to the file format that will produce the smallest image size (make sure you read the WARNINGS), and optionally apply lossy reductions for PNG and JPG images. Click here to download Ewww Image Optimiser
I hope this list helps, I am sure there are many more available on the market but these seem to be most popular. If you know of any better ones, please leave a comment!