Face vs X-face

[Pawel Salek <pawsa theochem kth se>]

Hi,

I have recently seen a couple of posts with fancy mugshots attached and  
I have thought about getting myself one of these, too. I do not know  
however how to go about it. Which one should I pick: Face or X-face?  
Does it matter which one I pick? When I click on the file selection  
button in Identities, I get to pick a file but I get somewhat cryptic  
message: "file is too big". How small should the image be for best  
results? Can it be color or should it be grayscale, or indexed, or  
black-and-white? Maybe balsa could/should reduce the file size itself  
if needed? Can anybody answer these questions? Answers in form of  
documentation patches get bonus points :)

Pawel