You can change the size of images, but it applies to both portrait and landscape images. The setting is called "Length of longest side of compressed images" and it is located in the Media Objects - Images section in the Site Admin area.

The best you can do is to reduce the size until the portraits are just right. The landscaped images will be smaller too, though.

Remember that users have different resolutions, so what works for you may not be right for everyone.

yes its the right way to do so...............

The best way to get up to speed on the source code is to upload an image and use the debugger to step through the code as the image is processed. Once you understand how GSP resizes an image you'll have a better idea of what needs to change to support your goal.