As you mentioned, the demo site is a great place for inspiration when trying to create a QV app, and even more if you never did one before.
When creating a QV app, there are many best practices to keep in mind like Design and User Experience, Data Visualization, etc. So, the short answer again is: check the demos if you are in a hurry. But if you have the time, try reading books and websites about these topics.
What I'm really looking for is a how to design for Qlikview book or user guide on Qlikview. The data crunchers are taking care of displaying data types I just need to be able to visualise how it could look. should i base this on a 990px wide screen, can I use jpeg background images, can I put in a custom graphic of a pie chart. Things like that. If it exists.
I'm afraid there is not a book about just Design for QlikView. There are some very good books about QlikView but they explain many aspects such data modelling, expressions etc. and not just about design.
But, yes you can use PNG, BMP or JPG images for background (and also for logos or images using the Text Object object).
The width depends on the resolution used by the users, but usually 1024px is recommended.
And I'm afraid you can't change the pie chart but you can use some kind of layering with Text Object if you want to make a chart look different, as seen in many infographics. I don't know if you are thinking about this but just in case I'll share with you this link to the blog qvdesign by Matthew Crowter.
Please see attached PDF.
Hope this helps.
There are several useful books you could get hold of.
QlikView For Developers by Barry Harmsen. An excellent guide to QlikView Development.QlikView 11 For Developers: Amazon.co.uk: B Harmsen, M Garci: Books
Information Dashboard Design by Stephen Few. Not QlikView specific but has some well considered thoughts on the best way to display data. Information Dashboard Design: Displaying Data for At-a-Glance Monitoring: Amazon.co.uk: Stephen Few: Books