I always pictured Breeze as a portly light-skinned black man.
As for Saze, I pictured him extremely pale-gray with stretched ears and a long face, with an air of prudent responsibility. Sazed is "the dutiful butler and scholar"
As for Vin and Elend, the covers sort of filled those two in for me. I liked Christian McGrath's interpretations as the writing confirmed.
I didn't really have any particular ideas about who the other characters would look like. The writing sort of spoke for itself. I have images in my mind but it's sort of hard to describe. There's always a character in every book whom I envision similar in appearance to myself, although with some changes due to the personality and morals of that character.