How to improve the performance of an ASP.NET application? Which are are the fields I should take care? The application includes DB connections and Image Parsing etc.
Use the .NET CLR profiler to determine where optimization will be useful.
http://msdn.microsoft.com/en-us/magazine/cc301839.aspx
The book Improving .NET Application Performance and Scalability has a chapter on Improving ASP.NET Performance, that might be worth reading. The full book is online at MSDN, and is also available as a PDF download.
I wrote a book about this that was just released, called Ultra-Fast ASP.NET: Build Ultra-Fast and Ultra-Scalable web sites using ASP.NET and SQL Server.
The book is about 500 pages long, so there's a fair amount you can do, at all tiers: client (HTML/JavaScript structure), IIS, ASP.NET, SQL Server, infrastructure, etc.
try avoiding unnecessary postbacks to pages, there are a lot of features you can implement by using things like jQuery or ExtJs framework. Learn how to do AJAX calls and pass info between your app and the server via JSON result sets.
Also, install apps like FireBug and YSlow and use these to analyze your application and follow their recommendations on how to speed up your app. Good luck!
10 Tips for Writing High-Performance Web Applications
20 Tips to Improve ASP.net Application Performance
Bye