Updated from the instructions in this post which seems to be a bit out of date now.
First off, please note that in order to display an animated GIF image as a camera, your Blue Iris must be version 18.104.22.168 or above. Images here are from Blue Iris 5.
The method below is used to determine the URL for an animated 'PNG' of the desired weather radar loop on Intellicast is as follows:
1) Go to this URL: Intellicast - Current Radar in your browser
2) Go to the dropdown menu to the left under 'Region' and select the desired region to be displayed.
3) Toward the right click on 'Play Animation'.
4a) In IE, right click on image and select inspect element to see URL. You are looking for something like this https://s.w-x.co/staticmaps/wu/wxtype/county_loc/sat/animate.png
4b) in Chrome just right click and select copy image address.
5) In Blue Iris, create a new camera configured as in images below using the URL for the PNG file obtained above.
Note it seems to not visibly affect the load on the server either.