Android Design: When should one load data (from a DB/or web service)

I'm currently designing an android app which will incorporate a contacts (friends) feature. This process is going to be tied to fetching from a database and getting pictures off the web.

I was just wondering what is the best/most used model to do this in android. Should all contacts be loaded from DB and then into the listview on the start of the app / should the data from DB be loaded at the start of the app and then the list view populated when one clicks the contacts buttons.

I'm still fairly new to android so if you guys have any tips it would be much appreciated.