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