Welcome!
For images: your best bet is to use a square image, 32x32 or 64x64 should be sufficient. It can be any image format that can be displayed as a background image.
For the FontAwesome icons: we are currently updating our FontAwesome library from 4.7 to 5 (more details here), and I will also add instructions as per below (this won’t be in master right away though):
