ADFS : You can change anything in the Theme structure

The official document around onload.js and customising the ADFS login page for 2012 R2 and 2016 (ADFS 3.0 and 4.0) is here.

The Theme directory structure looks like:

Directory of C:\...\Theme\Custom
05/11/2017 09:26 PM .
05/11/2017 09:26 PM ..
05/11/2017 09:26 PM css
05/11/2017 09:26 PM illustration
05/11/2017 09:26 PM images
05/11/2017 09:26 PM script
0 File(s) 0 bytes

Directory of C:\...\Theme\Custom\css
05/11/2017 09:26 PM .
05/11/2017 09:26 PM ..
05/11/2017 09:26 PM 8,144 style.css
05/11/2017 09:26 PM 8,146 style.rtl.css
2 File(s) 16,290 bytes

Directory of C:\...\Theme\Custom\illustration
05/11/2017 09:26 PM .
05/11/2017 09:26 PM ..
05/11/2017 09:26 PM 116,699 illustration.png
1 File(s) 116,699 bytes

Directory of C:\...\Theme\Custom\images
05/11/2017 09:26 PM .
05/11/2017 09:26 PM ..
05/14/2017 08:45 PM idp
0 File(s) 0 bytes

Directory of C:\...\Theme\Custom\images\idp
05/14/2017 08:45 PM .
05/14/2017 08:45 PM ..
05/11/2017 09:26 PM 931 idp.png
05/11/2017 09:32 PM 1,727 localsts.png
05/11/2017 09:26 PM 1,977 otherorganizations.png
3 File(s) 4,635 bytes

Directory of C:\...\Theme\Custom\script
05/11/2017 09:26 PM .
05/11/2017 09:26 PM ..
05/11/2017 10:23 PM 5,925 onload.js
1 File(s) 5,925 bytes 

The document refers to onload.js but you can use the same commands for any of the above files.

@Pierre e.g. has blogged on changing the "thumbnails" here.

The command for this then looks like:

Set-AdfsWebTheme -TargetName MyTheme -AdditionalFileResource @{Uri="/adfs/portal/images/idp/idp.png";path="C:\MyTheme\images\idp\idp-adfs.png"} 

Enjoy!