Via FAQ:

How do I make one of those cool banners for my plugin page?
Start by creating an assets/ directory in the root of your SVN checkout. This will be on the same level as tags/ and trunk/, for example.

Inside that assets/ directory, you can make a file named "banner-772x250.png" or "banner-772x250.jpg". The image must be exactly 772 pixels wide and 250 pixels tall. No GIFs.

To support high-DPI (aka "retina") displays, you can additionally make a file named "banner-1544x500.png" or "banner-1544x500.jpg". The image must be exactly 1544 pixels wide and 500 pixels tall. No GIFs. This image, if present, will only be served when the website detects that the viewing browser is on a high-DPI display. Read this feature's announcement post for more details and special notes about creating this image.

After you check in your new banner(s), it will appear the next time the plugin refreshes itself in the directory. This may take a few minutes.

For development and testing, you can add a URL parameter to your plugin's URL of "?banner_url=A_LINK_TO_YOUR_IMAGE" to preview what the page will look like with your own image. This will only work with your own plugins, you can not use this parameter on anybody else's plugins.