There is no bandwidth limit here at wordpress.COM. There are blogs hosted here that get hundreds of thousands of hits each day.
With a standard blog here you get 3GB of storage space, which is a lot of images and documents. The 5GB upgrade would give you an extra 5GB for a total of 8GB.
If you are wanting to use a lot of images, just make sure that you optimize them for the web. Since the search engines now consider total page size and page loading times when calculating search engine ranking, you want to keep things as small as possible. Also, there are still a lot of people out there on slower broadband (1.5Mbps or slower), so you have to keep them in mind as well. With the short attention span of web users now a days, they don't wait too long before clicking away and going somewhere else if a page loads too slowly.