Hi Amanda, Welcome to MobileRead.
Where exactly are you seeing this split jpg image? Is it on an ereader device or in the Calibre Viewer?
To the best of my knowledge when Calibre converts your html via a zip-to-epub or zip-to-mobi it uses the setting in Convert - PageSetup - Output profile to decide what to do with each image. If the jpg dimensions are larger than the screen resolution of the device in the output profile then the image will be rescaled downwards to make sure it fits the screen. I have never known Calibre to split an image into pieces.
It sounds as if you are creating your html correctly. Just try viewing the epub/mobi on the actual reader.
From my own experience (which is only with epub on Sony and Pocketbook devices) I have found it best not to include any width or height attributes in the <img> statement, i.e a typical <img> statement would look something like:
Code:
<p class="myimg"><img src="images/map.jpg" alt="Map" /></p>