This is sort of a newbie question, but I'm not a designer.
In iOS, we can make universal apps (i.e. support both iphone & iPad). But when it comes to background images' sizes I have NO idea what's the principle there.
For example, I have an idea for an app. It will be universal app. The login screen would have a full size image (or image that's stretched from size to size). The point is to make it look HD on different devices (from biggest iPad to smallest iphone)
Is there are some principles/rules to follow? What's the biggest size you make your full screen images?