Trouble downloading images

I LOVE this product and have been making great strides with it so far in my evaluation of the professional edition. I am having difficulties, however, downloading images.

SKU (6-digit number) and IMG_SOURCE (full http://.... URL of image)are tokens extracted from a DATARECORD. After each pattern application of the DATARECORD I am invoking a script which calls

session.downloadFile( session.getVariable( "IMG_SOURCE" ), "D:\Images\" + session.getVariable( "SKU" ) = ".jpg");

The "Images" folder exists and the images I'm downloading are JPG's. I'm trying to save each image with the filename of the SKU.

In my log I am ecountering:
The error message was: The application script threw an exception: java.lang.StringIndexOutOfBoundsException: String index out of range: -2 BSF info: null at line: 0 column: columnNo

I don't know how to interpret this. I've browsed the entire discussion board and your support documents, but nothing so far has provided a solution. I'm getting desperate! Any help is greatly appreciated.