Image formats and resolution – for Low Bandwidth Web Apps

I have a Business Requirement for remote place system (app) accessibility of the web application.

More specifically, The user of this application should be able to access this application from remote place where internet speed is limited (with 56kbps speed). Major part of the application with user engagement is filling up an online form and placing few service requests. And, a small part of application has images.

Now, I am not sure on technical specs of the image to be used in such an app to meet the Business Requirement. (When I say technical specs - size, format, resolution) I am looking if there is any suggestion, or guidelines to get this through.