Not sure if this is possible but it would be great to be able to assign an image (or images) to a specific portfolio category during the import process. This would be at the stage where you input title, alt-tag, etc and hopefully would be available for old images.
In this way, I presume that I would be able to use the same image in:
1. The initial blog post
2. A portfolio gallery using the category to pull it in
Since both link to the same JPG file, it would also make for a smaller use of space rather than what I understand is the current practice of uploading the same image twice. If this is possible, the next best steps would be a way to do this in bulk and/or convert tags or categories to a portfolio category for an image.