Hi, I have spent the past couple of days

trying to work out how to insert an image into the product description area.
I already have the product image up and running but would like to include another imager further down in amongst the description.
Is this possible?
I included the following in the product description area:
<img src="threesizes.jpg" alt="One Nappy - Three Sizes">
All i got was a rectangle with a blue question mark in it instead of the picture.
What am i doing wrong?
Thanks for any help!