When uploading your images to scenes pay close attention to the scene. Many of them will have the image dimensions written on it, like the picture ones here:
Logos are a bit trickier, for any of the round scenes a square based logo is best and remember when using the circular crop function there may be a buffer from the circle so your logo will appear smaller again. For other logo scenes a horizontal logo works well. It should be 1000px wide for either scenario.