Hello @loano1
Thanx for using our plugin
If you go to settings page and set it image size as max as shown here
You will get following results automatically and you need to understand that AMP is for mobile screens and not Desktop screens so this is what will happen when you set image to max size , see here
There is no need for any Custom CSS as this CSS style is added to all images , Look here
Therefore from all the above discussion it is clear that images in AMP are responsive we just need to set them to max size or any custom size so that all your target devise screens are Covered and they stretch to the entire width of Post
If you need any more Help please let me know ??
Regards
AzMak