When the module displays images in a small size, please tell me how to keep the rate between width and heght same. I know that we can use tool such as Paint to stretch or skew it. But I also see that we can do it by code. Please tell me how to do it.

Thank in advance.

Take a look at my open source DNN Forge project "Image Editor Control" which is available at www.codeplex.com/imageeditor. In particular, study the source code in the file ImageUtilities.vb. There you will find a wealth of methods for loading, saving, resizing, cropping, rotating, and otherwise enhancing bitmapped images. If you are looking to dynamically resize images and output them to the response stream, you will find many (similar) methods in the code for the DynamicImage.ashx HttpHandler.