Hello Ronak,
To make a banners for your plugin page, you have to :
1) creat an “assets” directory in the root of your SVN checkout
2) inside that assets directory, you can make a file named “banner-772×250.png” or “banner-772×250.jpg”. The image must be exactly 772 pixels wide and 250 pixels tall. No GIFs.
After you check in your new banner, it will appear in a few minutes