If a picture doesn’t show and in it’s place is a little box that almost always means you are NOT pointing correctly to where the file is located.
You’re not looking at the source if you get choices like small, med or large, sounds like you’re in the media center or somewhere else.
A link to a image should look something like this:
<img class="aligncenter size-full wp-image-7793" alt="One-step-at-a-time" src="https://www.be-awesome.org/wp-content/uploads/2013/02/One-step-at-a-time.jpg" width="600" height="400" />
Before changing either the width or height open file in some program like Photoshop so you can accurately determine the correct ratio if you make it larger or smaller… don’t guess.
Some themes are finky and require you include a full URL like above while others may allow a relative addresses.
A beginner mistake and one even experienced users make all the time is getting the path wrong. Just one little typo and you’re out of luck.
Translated to people talk the above link means the site is https://www.be-awesome.org, the wp image # is assigned by WP, don’t worry about it or change it. The wp-content/uploads/2013/02/ is a pointer to where on your server the file is and finally the full name of the file. One-step-at-a-time.jpg: Spacing and/or if or not there are caps are important and must be exact on many servers.