Easy.
Where you want the image to appear, simply type<img src="URL">.
For example, placing <img src="http://www.pispeakers.com/photos/Stage_four_Pi.jpg"> will give this...
Type "basic HTML" into a search engine and you will get lots of hits for pages that tell you how to do this and lots of other things in HTML. Plus I'm sure there are others here who know lots more about this than me and can explain it better.
HTH