It's pretty simple.
1: Get any program that modifies images such has paint. (I recommend a program that has transparency options such has Gimp)
2: Save your logo into a 128x128 .png file somewhere you will remember.
3: Put it here : \Steam\steamapps\common\Production Line\data\bitmaps\logos
4: Change the file extension from .png to .dds
5: Boot the game and your are done.


PS: This also works for floors. I haven't tried walls yet nor cars.