Hi yogesh,
You could hide it even if it couldn't be customized.
(search CRM welcome popup)
Here are two methods, 1 is from stackoverflow, 2 is my thought.
1. Create a dashboard with a large HTML web resource and set it to default.
2. Still create a HTML web resource with custom image, insert a small web resouce with only javascript to your current default home dashboard,
run parent.Xrm.Utility.openWebResource() function in the small web resource to open the large as custom popup.
Regards,
Clofly