Data of models (images included) is stored in a file name Models.cached
(in the GameData/Models folder)... when you change something in models, like the picture, destroy this file...it will be recreated in your next run of the game and by then will take your latest changes, like pictures.
To add bios to leaders, there must be an entry in the string file which, in column A, starts with ldr_txt_
This is how it works in CW2 for instance.
This entry is absent, I believe it was not planned at all by the designer to have bios. You can add then yourself if you wish, just enter them at the end of the file. In addition, so generals, in the Models DB, have the entry ldr_txt_NoBio
... which means it was a deliberate choice not to ever have one (e.g. fro Dragomirov).
Al this can be changed, but it will take a while to do so