This article provides information on the recommended image sizes for various image functions in TeamSideline:
Background Image - 2000 x 2000 pixels
Page Banners - Width: 1130 pixels. Height can vary but make all images in a given Rotator the same height.
News Items: 180x150 pixels
Public Badge: 300x200 pixels
Portal Badge: 180x120 pixels
Favicon: 192x192 pixels