I may be wrong but I don't think webtools comes with a facility to automatically substitute a 'default' image ' no image available' when the product image does not exist. Please correct me on this!
I have therefore tried to code up a method which populates a default image in this case but so far without success because of the hierarchy of the page generation process.
[the following would function fine normally but does not work at the correct point in the page lifecycle
this.imgpart.ImageUrl = _bp.AssetsPath.ToString() + "catalog/parts/noimage.gif ]
Incidentally I have noticed that if you wish to create additional links to files other than images in the product media table the hyperlinks fail unless the product image exists so there is a double reason to want to ensure that all products have an image file.
Does anyone have any suggestions?