For me it is usually either a spatially disconnected weak image not anchored anywhere, or when I try to picture things within my field of vision it is like my imagination is being shown as a reflection in glass, like when you look out through a window at the real world and see your room faintly in the reflection, where I see my imagination like that reflection.

The strength of it varies with my focus and general mental state (it is easier when I'm relaxed).