How to use custom portraits?

So I've been trying to get custom player portraits to work in 2K18 but have had no luck. I've watched both YouTube vids on it so I understand how to do it but saving the file is the issue. I was following the one video with paint.net but when I go to save it as a DDS file it doesn't correctly save. Instead of turning into that blank DDS file it stays as an image and instead of saving as DDS it saves as a paint.net file. I've tried to download that intel file from the other post and rename it as DDS2 but none of it works. Any ideas on how to fix this?