I made an app/channel from the videoplayer example in the SDK. It had a sample image called MainMenu_Icon_CenterFocus_HD.png which is 366x210 (ratio 1.74). I replaced it with my own logo now on the "My Channels" page on my Roku3 it looks squashed. It can be seen here: https://www.dropbox.com/s/sqp8aakry57cq ... 1.JPG?dl=0 (I put the little red "1" to debug to myself which images gets used)
If you're looking at the icon with the channel side-loaded, it will look squashed because the side-loaded channel uses the icon in your package. Once you create a public or private channel and upload your package, it will use the Channel Store icon.