Creating and Publishing your custom icons for CRM entities makes your solution that much more professionally-looking.
There’s two ways to load your custom images onto CRM:
For On-Premise only:
upload your images in the CRM web folder called _imgs (default installation is at C:\Program Files\Microsoft Dynamics CRM\CRMWeb\_imgs)
For both Online and On-Premise:
make sure you add the images as web resources in a solution