As we have said elsewhere in this documentation set, Wordpress is a smart system!  In the world of computers, smart also means following the owner’s rules.

So, if you try to upload a picture and use it in a post/page. You will see that wordpress allows you to use the existing size of the picture (No changes will be done) and in addition allows you to reduce the picture size to another 3 levels from small to large.

Once you choose to resize the picture, Wordpress will keep the image proportions and will use pre defined sizes. Those sizes are configured in the Media section.

Note that the sizes in Pixels.

This is the section where you can set the standard sizes for images for your site and Wordpress will automatically scale for you.  All you have to do is select a size (small, medium or large) when inserting the image and wordpress will do the rest – according to your rules set here.