The only way to disable this is to alter the CTM file settings to remove the mapping from the attributes for width and height. This is typically done using the XMetaL Developer CTM Editor.
Here is an example from the Journalist demo included with XMetaL Author:
Removing the values in red will disable the automatic insertion of the width and height. Note however, that this also disables the feature that will resize the image based on those values, so if you want to override the default (ie: 100%) size (obtained by reading the content of the bitmap image) you must leave these alone. This will not be an issue if you always want images to appear at their default size during editing. This might also be less of an issue if you don't even allow your end users to modify these values (which is easy with XMAX because an Attribute Inspector is not provided, and if you do provide one you can code it so that these attributes are not available).
