For apps hosted on Splunkbase, the app icon needs to be included within the package. The app icon displayed on Splunkbase is used by the default version selected. For more information:
http://docs.splunk.com/Documentation/Splunk/latest/AdvancedDev/AddConfigurations#Add_app_icon_images
For apps hosted externally, the Splunkbase app icon can be managed from the app edit page. However, note that the icons will still need to be included within the package for them to be displayed within Splunk.
http://dev.splunk.com/view/SP-CAAAE86#icons
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