If you are trying to specify the image (icon) that is shown on the plugin's page, then try naming the image file icon-128x128.png/jpg .
Banner images are
Normal Banner: banner-772x250.(jpg|png)
Normal Banner (RTL): banner-772x250-rtl.(jpg|png)
High-DPI (Retina): banner-1544x500.(jpg|png)
High-DPI (Retina RTL): banner-1544x500-rtl.(jpg|png)
If the banner (or icon, different things, I know) doesn't show up on the plugin listing page, perhaps the updated info hasn't been synced.
You could try changing the version number in the plugin and readme to see if that will update things. Don't know if just uploading/svn-sync the assets folder will change things on the plugin settings page.
Also, this post seems to indicate that you need two versions of your banner image to support retina displays: