In Moving Pictures, video-cameras are invented... But they're essentially the same thing as Twoflower's camera which only took one picture at a time but Twoflower probably could've asked it to paint many very, very fast. So did these character's not know about these iconographs? And if so, why didn't merchants from wherever Twoflower is from sell the people of Ankh any of their cameras?
Devices like Twoflower's iconograph seem to be fairly common in Ankh-Morpork after he introduced the technology to them, and we see them used extensively both by the Watch and the newspapers in later books. The movie cameras created during Moving Pictures use the exact same process, with imps rapidly painting pictures. However, the thing that makes film possible is the invention of "octo-cellulose", a transparent medium that the imps paint on which light can shine through.
These six demons here...look out through the little hole in the front of the box and paint pictures of what they see.There has to be six of them, OK? Two to paint and four to blow on it to get it dry. On account of the next picture coming down, see. That’s because every time this handle here is turned, the strip of transparent membrane is wound down one notch for the next picture.
The origin of the movie camera seems to have been the spirit of Holy Wood; the Boke of the Film tells us that cinema existed at some point in the distant past, but we don't know if it used iconograph imps.
It seems likely that once Twoflower's iconograph became known various artificers will have heard about it and tried to replicate it. When an alchemist who knew about octo-cellulose found out about it that led to the creation of a practical movie camera. Its not clear from the book how much of this was human ingenuity and how much was due to suggestions by the roving spirit of Holy Wood.
As for the last question, at the time of Moving Pictures the Agatean Empire had a closed border; international trade was strongly discouraged.