The RESTful Map API (a.k.a.. Map Image API) is covered by the terms of agreement defined here: http://www.developer.nokia.com/Develop/Maps/TC.html
The relevant clause seems to be 4(x):
As a simple request-response service, I don't think the RESTful Map API offers a caching service directly, but some of the other APIs do. If your mobile application uses the Maps API for Java ME for example, caching is provided out of the box:
When using the APIs you hereby agree that You will not
4(x) pre-fetch, cache, or store any Nokia Maps Content or Places Content except through API functionality specifically intended for such use and except that You may store (i) references and IDs; and (ii) limited amounts of Nokia Maps Content or Places Content solely for the purpose of testing your Application, if You do so temporarily, securely, and in a manner that does not permit use of the Nokia Maps Content or Places Content outside of the Service;
So it looks like accessing the Images properly online and then placing into some sort of limited local storage is permitted, but taking a copy of an image and holding as a PNG resource wouldn't be acceptable.
map.setCache("CachedDemo/cache", no_items, min_free_percentage);