You're exactly right: I know what a Sphere looks like but I am missing the bit that the rest of the world seems to have where they can conjure up an image of it. Even less abstract objects like a desk or Angela Merkel's face, I am quite unable to picture.
To answer your question, if I ask you to picture a barn in a field, you presumably have no difficulty in doing so. You've probably even made assumptions about things like the colour of the barn, the weather or surrounding terrain and have a sort of portrait with a barn at the centre, but I can't do that. I am having difficulty describing what I get but it's nothing close to anything like that. I know my barn is made of wood and I know it's in a field but that's about as far as I can get.
I'm happy that this is getting discussion because obviously it's quite strange to find out that a) you're not wired the same way as the majority of the world and b) that your self-diagnosis isn't a medically recognised condition. I'm excited to see what research comes out over the next few years.