When using PageDown, there is some problem when inserting an image which is too big.
It is not redimensioned to a size that would fit in the editor's width (that would be possible with <img src="" width="...">
)
Example : try to insert this image URL http://upload.wikimedia.org/wikipedia/commons/4/4b/Apple_pie.jpg in this editor :
http://www.markitdown.net/markdown.
How to make image fit in the container with Markdown ? (this is done on StackOverflow's editor)
When working with the Pagedown editor
http://pagedown.googlecode.com/hg/demo/browser/demo.html
it finally works by adding
#wmd-preview img { max-width: 500px; }
All images (even big ones) will have a max width of 500px.