Are screenshots really that useful or interesting? I'm not being snarky, I'm genuinely interested.

99% of the software I've written has been embedded deep inside some expensive piece of machinery. A visual portfolio would be useless since it would consist of photos of the machine, which would offer no insight. Instead I describe what I've done, how it works, problems I had to overcome, etc.

I've interviewed people for many types of positions, and I can't think of a single time when the ability to answer questions about the software they've written wouldn't have sufficed. As it is, I rarely even get that :-(