If you want to get an image to appear inside the info window of a marker, the first thing you must do is obtain that images URL.  When configuring a marker, you have the option to specify an image url.  This should be the absolute URL of the image.  Relative URL referencing might work but I haven't tried it and will like yield unpredictable results.

Once you know the URL, then do the following steps:

1. Associate the URL to the marker you want the image to appear on by editing the marker information.
2. Edit the map where this marker appears.  Displaying of an image is configured at the MAP level only.
   
3.  Select the 'Markers' tab from the Map Edit page
4. Turn "Enable Driving Directions" to YES.  (NOTE:  This really should just read Enable Tabs)
5. In the "Tab 1 Content"or "Tab 2 Content", you need to change the format to either Details with Image or Image only.
6. Save the map and see what happens.

Just always remember that how the marker information pop-up windows are displayed is configured at the map level.  If you set the Tab 1 Content to "Details with Image" and the marker does not have a URL, then the display will appear like a standard image only.