PNG File
from
PolMach Forums
What is a PNG file? And how doyou make your pic a PNG file?
Nevertheless, use a .png for your additions. 
2.4. Custom character portraits
-You can insert your own portrait into the game for use as a custom character.
Create a 211x256 pixel .PNG file with your character portrait and
save the file to the C:\Program Files\Ubisoft\Stardock\PolMachine\Gfx\Candidate
directory.
For example, saving the file with Adobe Photoshop, you should save
a 24-bit .PNG with transparency to the above directory.
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