cs:go minimaps are in the .dds format.
This is the plugin I used to save my one, so if your using photoshop here is the link:
The files have to be located in csgo/resource/overviews/.
So basically you follow that tutorial up to the point where you create the vtf and stuff, but instead you make a .dds file.
You need two files in your overviews folder for each map.
The txt should contain the regular stuff :P
"material" "overviews/de_examplemap" //no _radar on the end this time.
Files Pakrat fine. The txt file will be picked up automatically, the _radar.dds file will have to be added yourself. (make sure you dont leave your whole computers url in)
If server owners want to use maps that don't have pakrat'ed minimaps, then check out this plugin.
[CS:GO] Radar and loading screen downloader
Ok, hopefully I wrote it down all correctly :P I did get it working myself.
EDIT: Forgot the plugin link, and updated the post in general.