you'll find something looking like:
the easiest way of making it a dynamic width is to do this:
<img src="directory/imagename.jpg" />
or making it a fixed width (average resolution is 1024x768 so I will do it for that resolution for you to fit nicely.
<img src="directory/imagename.jpg" width="80%" />
I think this is what you are after, and I will note that this doesn't ACTUALLY resize the image it only makes the original image squashed, if you wish to actually resize the image I would suggest you export it at a smaller size somehow, as the alternative is quite advanced if you are at an introductory level.
<img src="directory/imagename.jpg" width="950px" />
Hope it helps!