If there was a whole civilization of people without sight, how would they imagine things? Would they be able to picture 3 dimensions in their heads by feeling things?
Of course. After all, blind people in real life have been composers, painters, and authors. Most things in life have multiple sensory aspects. A blind person could sculpt by feeling the geometry of their medium (for example, Felice Tagliaferri). A blind artist could paint by knowing the relation of one line to another, which is likely proprioception-based. Such an artist could also lay down a textured surface as a guideline. One example would be John Bramblitt (https://en.wikipedia.org/wiki/John_Bramblitt). Composing music, an imaginative art, is of course scarcely more difficult for the blind individual than for the sighted(compared to the previous arts). Perhaps a few people here have heard of a guy called Stevie Wonder? (https://en.wikipedia.org/wiki/Stevie_Wonder). A blind person can even fly a plane (http://planelopnik.kinja.com/blind-pilot-flies-and-lands-a-real-airplane-1657930420). Talk about destroying stereotypes. I imagine that for someone who can conceive of the position of the plane relative to the ground, flying blind presents relatively few obstacles (though keeping it level might be hard).
All of the previous examples were merely for modern human societies. Since this question is in Worldbuilding, perhaps non-human senses or advanced technology is permitted. In that case, there are so many more possibilities for sensing 3D geometry: echolocation, thermoception, telepathy, magnetic fields, air currents, even whiskers!
Bat's have extremely limited sight, but they probably understand 3-dimensional space better than we do.
As a real world example, composers imagine/design new songs.
They wouldn't associate it with "sight", but instead with another sense - probably touch.