Insight icons are used to give object types a graphical identification. You can upload new icons or delete existing ones. You also have an option of Themeable icons whose colors can be edited to suit your organisation's brand.
Be sure to upload images bigger than 64 pixels.
As an Insight Administrator, you can configure icon changes both on general Insight configuration as well as a specific schema configuration. You can read about it here.
As an Insight Manager, you can configure icon changes for your own schema only. You can read about it here.