On: Show HN: Our 5 month project Airtime for Email

But wouldn't a text signature be many times more effective then an image that may not even appear in some mail clients?

I don't know the answer. It might. In general, graphics have better CTR (at least in my industry, which is not as tech savvy), but you're correct that image blocking counteracts that to some degree. It's worth doing a trial and seeing which performs better.