Setting Up Images For Preview in nvAlt
These are notes from: here
Best Method
Update the HTML preview template at:
~/Library/Application Support/nvALT/template.html
to add this line in the HTML <head></head>
<base href="file:///Users/alans/Dropbox/grimoire/_images/">
Note the trailing slash is required to get it to work right.
This will let you post image like this:

To prevent alt text from showing up in the preview, simply leave the square brackets blank.

Alternate Methods
Drag and Drop
If you drag and drop an image on nvAlt, it'll create a reference with the proper path filled out.
Full Path
Putting the full OS path to an image works. For example:

Per-File Base Folders
You can add header info to a nvAlt File. The image root path for any given file can be set this way. The following has to be the first line with a single empty line after it:
XHTML Header: <base href="file:///Users/alans/Desktop/">
-- end of line --