First off, you must make a profile field.
Do this by logging into your AdminCP and going to, User Profile Fields > Add New User Profile Field
Fill out the info as needed. No specifics really.
Next you must edit your postbit or postbit_legacy template.
Again done in the AdminCP. Styles & Templates > Style Manager > Edit Templates > postbit_legacy(or postbit)
Change the red to the profile field number.
Change the green to the url of the image you plan to see on your postbit.
Change the blue to whatever you want the image to link to.