How to transfer IPB 2.1 template statics to CDN for a part of users by IP?

Question

IPB

I want to transfer the statics of the IPB template from the server to the CDN
. It is complicated by the fact that there are two groups of users - from the Internet and from the local network.
Determine who from where I can by IP address. For users from the local IP of this kind 10.xxx
I see two solutions:
1 in the template code, depending on the IP address, change the path.
I think this option is more correct, but I could not implement it.
Suppose the desired image is displayed through '<{C_ON_CAT}>'
How to attach to this tag to redefine the path to the image?
Doing str_replace throughout the page code after the template engine has completed is not the best option.
2 put htaccess in the right folders to redirect users from the internet.
Not sure if it makes any sense at all. Is it worth it?