Hey all,
I did a quick test, and putting any .png in in the location Jpro600k mentionned with the correct sound designer name seems to display it in Pigments:
C:\ProgramData\Arturia\Pigments\resources\sound_designers
/Library/Arturia/Pigments/resources/sound_designers
So simply make sure your presets have the same sound designer name as the picture.
EG: The image called My Name.png will be displayed in all presets containing "My Name" in the sound designer field
Make sure the size is 324x252 to avoid cropping
Best,