Welcome to the Ranch.
This is a very interesting problem. I am imagining it on the lines of "image data pagination".
Purely from the usability perspective, I would say this is not really a user friendly feature. In case of maps, the end user does have an idea that something exists beyond the displayed map. So it is sort of intuitive to drag/move the map. I am not sure it would me similar for images. The only flip side would be you can save download times. But I think that is where the problems would start.
I am not aware of any library or build in GWT class which would allow you to do this, nor have I done anything like this previously. However this is how I would approach it, if it were my problem
1) Somehow find a way to tweak the graphics and canvas on the client side, and render the image data on it
2) Have some code on the server side which will accept the start x,y co-ordinates along with the view dimensions(width, height). Then it would extract the corresponding data from the big image and send it back to the client over the network (either as sub image or byte[] or whatever is convenient)
3) On the client side, obtain/convert the sub image and render it on the UI
4) Listen to user view/drags and keep updating the view.
I remember reading about some graphic/canvas manipulation libraries for GWT. A quick google gave me
http://gwtcanvasdemo.appspot.com/ I am sure there might be more.
On the server side, you need to be careful on how you obtain the sub image from the big image. It could be a bottle neck if you load the big image everytime you want to obtain the sub image.
Caching might be a good idea both on server as well as clients. It would help performance when user drags back or overlaps.
