If you are doing one picture on its own line, it's better if it's a wider/horizontal type of picture. Quality-wise, it should be at least 350 pixels width.
Add the IMG tag after paragraph or list's closing tags (</P> or </UL>). Inside IMG tag, also add
See how big the picture is.
...set the width down to maximum "500".
Before/after the IMG tag, add CENTER tags. Make sure it's closed, turns purple, or everything else on your page will suddenly become centered!
If you have 2 square/tall type pictures, you can have them together on one line. The key is to set the height to the same amount (instead of width).
If you like, you can add some horizontal space of 5 pixels.
You can even have 3 pictures centered (if you have more than 6 pictures on your page). You can see the order: left/first picture on page = last IMG tag in code.