I just tested it.
It seems to me that all the images came.

Your API Request is this:

https://api.themoviedb.org/3/tv/71912/season/1?api_key=THEAPIKEY&language=en-US&append_to_response=images

Mine is this

https://api.themoviedb.org/3/tv/71912/season/1?api_key=THE_KEY&language=en-US&append_to_response=images

EDIT
OK. Understand.
In fact, it does not return the images.
I do not know why.
Let's wait for Travis Bell to come by and tell us what the problem is.

EDIT 2
Apparently if you do not specify language-REGION the images will return.

https://api.themoviedb.org/3/tv/71912/season/1/images?api_key=THE_KEY  
https://api.themoviedb.org/3/tv/71912/season/1?api_key=THE_KEY&append_to_response=images

EDIT 3
If you want to keep the language-REGION specification, then you need to specify the language of the images you want using the parameter &include_image_language =

https://api.themoviedb.org/3/tv/71912/season/1?api_key=THE_KEY&language=en-US&append_to_response=images&include_image_language=en,pt,hu,null  

More information about this parameter, read here:
https://developers.themoviedb.org/3/getting-started/image-languages

Yes, the solution is to add include_image_language:

https://api.themoviedb.org/3/tv/71912/season/1?api_key=###&language=en-US&append_to_response=images&include_image_language=en,null

If you wanted the images that haven't been tagged, and those tagged as English. This is all outlined in the docs here.