Follow blogpost if you are using Storefront version 2.6 or below:
https://www.citrix.com/blogs/2013/08/21/xd-tipster-changing-delivery-group-icons-revisited-xd7/


In case of Storefront version 3.0 or above StoreFront now provides its own desktop icons in preference to those received from the server. However, you can get back the old behaviour by using the following PowerShell (adjust IIS site ID and virtual path of your store as appropriate):

& 'C:\Program Files\Citrix\Receiver StoreFront\Scripts\ImportModules.ps1'

Disable-DSStoreSubstituteDesktopImage -SiteId 1 -VirtualPath /Citrix/Store

This needs to be done after changing the IconUID and all.